Part 1 of mega-change in #578

Major notes:
- Input(...) and Output(...) now (effectively) recursively override their elements' directions
- Nodes given userDirection (Input, Output, Flip - what the user assigned to _that_ node) and actualDirection (Input, Output, None, but also Bidirectional and BidirectionalFlip for mostly Aggregates), because of the above (since a higher-level Input(...) can override the locally specified user direction).
- DataMirror (node reflection APIs) added to chisel3.experimental. This provides ways to query the user given direction of a node as well as the actual direction.
- checkSynthesizable replaced with requireIsHardware and requireIsChiselType and made available in chisel3.experimental.

Internal changes notes:
- toType moved into Emitter, this makes the implementation cleaner especially considering that Vec types can't be flipped in FIRRTL. This also more clearly separates Chisel frontend from FIRRTL emission.
- Direction separated from Bindings, both are now fields in Data, and all nodes are given hierarchical directions (Aggregates may be Bidirectional). The actualDirection at the Element (leaf) level should be the same as binding directions previously.
- Bindings are hierarchical, children (of a, for example, Bundle) have a ChildBinding that points to their parent. This is different than the previous scheme where Bindings only applied at the Element (leaf) level.
- Lots of small misc clean up.

Future PRs will address other parts of #578, including stricter direction checks that aren't a side-effect of this internal refactor, stricter checks and splitting of binding operations (Wire vs. WireInit), and node operations not introduced here (getType and deprecation of chiselCloneType). Since those shouldn't mess with internals, those should be much smaller.

Also make transform instantiation deterministic

bump scoverage version

Replace ambiguous bi-connect ("<>") with mono-connect (":=") for internal Pipe wiring.

* Update comments describing Decoupled/ReadyValid.
It seems there is a valid use case for EnqIO/DeqIO and updating the comments may clear some of the confusion and encourage their usage.

* Update comments - no functional changes.
Re-flow comments for ReadyValidIO()
Add gen param to DecoupledIO() and IrrevocableIO().

* Update code and comment now that #492 is resolved

Enables changing FIRRTL's IR to only accept multiplication of identical
types.

Fixes #495

Helps distinguish between Records/Bundles defined in Chisel._ vs.
chisel3._. Also override compilationOptions when bulk connecting
Records/Bundles defined in Chisel._. This allows Records/Bundles defined
in Chisel._ code to be correctly bulk connected in chisel3._ code.

* Remove explicit import of NotStrict - fixes #492

* Provide macro for MemBase.apply().

* Provide macro for MemBase.apply().
Since a macro cannot override an abstract method, provide a concrete
apply method n VecLike() that we can override with a macro.

* Remove concrete apply() in VecLike.
Since MemBase no longer extends the trait VecLike, we do not require a concrete method to which we can apply a macro to extract the appropriate CompileOptions.

* Add missing implicit compileOptions to do_pad() and do_zext().
The latter caused:
```
[error] /vm/home/jenkins/workspace/rocket-chip_with_chisel3/hardfloat/src/main/scala/MulAddRecFN.scala:205: too many arguments for method do_zext: (implicit sourceInfo: chisel3.internal.sourceinfo.SourceInfo)chisel3.core.SInt
[error]     val CDom_sExp = io.fromPreMul.sExpSum - io.fromPreMul.doSubMags.zext
```

* Add SourceInfoTransform macros to Vec methods in order to avoid apply() chain issues.
Since utils methods are no longer NotStrict, Pipe objects need access to the client's compile options. There may be more.

* Respond to review comments.
Don't propagate SourceInfo through helper functions.
Replace old usages of CompileOptionsTransform with the now equivalent SourceInfoTransform and redefine CompileOptionsTransform to only deal with CompileOptions.
Just thread CompileOptions (not SourceInfo) through deprecated functions.

* Remove explicit import of NotStrict - fixes #492

* Provide macro for MemBase.apply().

* Provide macro for MemBase.apply().
Since a macro cannot override an abstract method, provide a concrete
apply method n VecLike() that we can override with a macro.

* Remove concrete apply() in VecLike.
Since MemBase no longer extends the trait VecLike, we do not require a concrete method to which we can apply a macro to extract the appropriate CompileOptions.

* Partially revert 8e4ddc62

It was an incomplete fix for handling Vec(0).

* Fix assignment from 0-entry Vec: add test

375e2b6a introduced a regression for bundles
containing zero-entry Vecs.  Until zero-width UInts are supported, the
zero-entry Vecs need to be flattened out before doing asUInt/asTypeOf on
a bundle.  Undoing that commit's replacement of Data.flatten with
Aggregate.getElements is the best interim fix.

* still trying to find right mix

* Making some progress on Mux1H

* Mux1H that works in non-optimzed fashion for FixedPoint, works pretty well in general
Catches some additional problem edge cases
Some tests that illustrate most of this

* Moved in Angie's code for handling FixedPoint case
Cleaned up tests considerably, per @ducky64 review

* Just a bit more cleanup

Fixes #554

Using the sample_element of the created wire is incorrect because Wires have no
direction so the Wire constructed for a Vec of Module IO was constructed
incorrectly. Fixes #569 and resolves #522.

Make it relatively easy to override a single CompileOption.

Fixes #567

Creating FixedPoint literals was throwing away width

This caused one hot muxing problems in dsptools
FixedPoint spec fixed based on error uncovered by this change

This also allows asUInt/asTypeOf to work properly on those Bundles,
even though zero-width wire support is lacking.

Use fold(0) instead of reduce to handle the corner case.

Allow muxing FxP of different widths and BPs

* Revert "Change Vec creation to check if gen is lit (and hence needs to be declared)"

This reverts commit dc86e7e1734d6abacb739b488df1de231e6b41b2.
This may address #522 - using chiselCloneType (instead of cloneType) to preserve directionality.

* Add missing implicits to Vec.apply() signature.

* Use correct macro (CompileOptionsTransform) for indexWhere.

This is an odd one.  Using log2Ceil directly results in a Verilator
compile error, presumably due to a FIRRTL zero-width wire bug.

The old implementation failed to check for width <= -2, and
did the wrong thing when -1 was explicitly passed.  Splitting
into two methods avoids the latter issue.

log2Ceil's argument might be 1, so employ a max operator.

Since the argument is at least 2, this change has no semantic effect.