Make Driver a ChiselStage compatibility layer

This converts the original chisel3.Driver to use
chisel3.stage.ChiselStage. This works in the following way:

  1. ExecutionOptions are converted to an AnnotationSeq
  2. The AnnotationSeq is preprocessed using phases contained in the
     FIRRTL and Chisel DriverCompatibility objects.
  3. ChiselStage runs on the preprocessed AnnotationSeq
  4. The output AnnotationSeq is "viewed" as a ChiselExecutionResult

This modifies the original DriverSpec to make it more verbose with the
addition of info statements. The functionality of the DriverSpec is
unmodified.

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