Before, was always deleting registers

Bug was if a register has no initialization, it initializes to itself,
which ends up preserving the register from getting eliminated.

The previous version basically eliminated nothing, because it consider
both the driver and the load to be uses.

Signed-off-by: Adam Izraelevitz <azidar@gmail.com>

Use newer rocket regression spec without comb loop

HTTPS clone is with .git; cd only with no error.

SInt representation is no longer 2's complement, but instead a positive number (hex or base 10) that is optionally preceded by a sign (-+).

* Added Zero width wires.

Semantics:
 - No change to width inference rules, e.g.
     a<0> + b<2> = c<3>
 - Replace zero width wires with UInt<1>(0) or SInt<1>(0)
 - Performs constant prop.
 - Redo width/type inference

* Remove errant println

* Moved ZeroWidth after ConvertFixedToSInt

* Added more tests, bugfix match on connect

Also replaced constprop with infertypes for correctness

* Updated to new emitter and test infrastructure

Now, any annotation not propagated by a transform is considered deleted.
A new DeletedAnnotation is added in place of it.

Changes Emitters to also be Transforms and use Annotations for both
telling an emitter to do emission as well as getting the emitted result.
Helper functions ease the use of the new interface. Also adds a
FirrtlExecutionOptions field as well as a command-line option. Use of 
Writers in Compilers and Emitters is now deprecated.

Workaround for #470. This allows parsing DoubleLits in subfield
expressions.

* castrhs shouldn't assume rhs is uint

* don't cast if types are the same

* changed castrhs to catch invalid lhs, rhs type combinations

* change error msg