Constant Propagate dshl and dshr with constant amounts

Fixes #990

h/t @pentin-as and @abejgonzalez

This fixes issue #988
I tried one alternative to this fix: record the time to do a *no rename* run (`depth = 0`) and check that the time to do the *deep rename* (`depth = 500`) was a reasonable multiple of the *no rename* test. Unfortunately, the discrepancies were all over the map, sometime as much three orders of magnitude difference.
I decided the current fix was the simplest - don't enforce timing checks if we're doing coverage testing, although determining the latter is brittle.

Previously, components that did not affect the output would cause
exceptions because they were missing from the label2group Map. This
commit treats them as "reachable" by the ports so they are included in
the default "ungrouped" group.

This fixes an issue where expressions created by GroupComponents would
be improperly lowered because they were not marked as references to
instance ports.

* Seal Direction trait

* Add WRef factories for ports and instances

Fix NoneCompiler outputForm

This adds a requirement that all Compilers must have at least one
Transform. Without this, there is no way to determine the inputForm or
outputForm of a given compiler as these are (rightly) defined in terms of
the head/last transform.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>

This changes the NoneCompiler to be a unary sequence consisting of an
IdentityTransform. This fixes the inputForm and outputForm inherited
methods that implicitly mandate a non-empty transform sequence.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>

This adds an identity transform that applies an identity function to some
CircuitState, i.e., it just returns the original CircuitState. This is
useful for transform generators that may, for edge cases, generate an
empty transform sequence. Other classes (e.g., Compiler) have explicit or
implicit requirements that a transform sequence is non-empty.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>

* Give better error when mport references non-existent memory
* Closes #796

Remove side effect from DiGraph sum

This fixes a bug where DiGraph summation (using the `+` operator) would
mutate the DiGraph. This occurred because the underlying edges set was not
being cloned. This is fixed to explicitly clone the underlying edges set.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>

Iterating on a HashSet could cause identical modules (including
annotations) to not dedup

* Added Foreachers

* Changed CheckTypes to use foreach

* Check widths now uses foreach

* Finished merge, added foreachers to added stmts

* Address reviewer feedback

Where the high form compiler removes Chirrtl (and runs some checks),
this compiler does nothing but read in the circuit and then emit it

- Change firrtl.options API, add Phase

This breaks firrtl.options.Stage into a small type hierarchy:

  * Phase: something that transforms an AnnotationSeq
  * Stage extends Phase: a Phase with a Command Line Interface

Some of the old "common options" (input annotation file and target
directory) are moved into firrtl.options and provided as part of the Stage
class. Stage will automatically preprocess an input annotation sequence to
resolve all input annotation files and add a default target directory.

Minor changes:

  * Adds ViewException
  * Stops mixing in the DoNotTerminateOnExit trait into the default Shell
    parser
  * Add StageOptionsView

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>

- Remove firrtl.altIR package

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@gmail.com>

* Bump sbt to 1.2.6; update dependencies

* Remove left-over surgery items.

* Add explanation for explicit junit library dependency

Huge performance improvement when you have lots of ports for a given
module. Also split up some long implicit argument lines.

- Add firrtl.transforms.CombineCats
- Use CombineCats in LowFirrtlOptimization
- Modify Verilog emitter to allow for nested Cat DoPrims
- Modify firrtlEquivalenceTest to write input FIRRTL string to test directory

- Add deprecated TargetDirAnnotation type to firrtl package

TargetDirAnnotation was moved from firrtl to firrtl.stage. However, this
is only aliased as a val in the firrtl package object. This also needs to
be type aliased for matching against a type. This fixes a bug I ran across
in the visualizer.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>

- Add, but do not use Options-mirroring Annotations

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>