Add FirrtlStage, make Driver compatibility layer

This adds FirrtlStage, a reimplementation of the original FIRRTL
Driver as a Stage. This updates the original firrtl.options package to
implement FirrtlStage (namely, TransformLike is added) along with
FirrtlMain. Finally, the original FIRRTL Driver is converted to a
compatibility wrapper around FirrtlStage.

For background, Stage and Phase form the basis of the Chisel/FIRRTL
Hardware Compiler Framework (HCF). A Phase is a class that performs a
mathematical transformation on an AnnotationSeq (in effect, a
generalization of a FIRRTL transform). Curtly, a Stage is a Phase that
also provides a user interface for generating annotations. By their
construction, Phases are designed to be composed sequentially into a
transformation pipeline.

This modifies the existing options package (which provides
Stage/Phase) to build out a type hierarchy around Stage/Phase. This
adds TransformLike[A] which implements a mathematical transformation
over some type A. Additionally...