Add chisel3.stage.phases.Convert Phase

This coalesces three distinct operations into one Convert Phase:

  1. Chisel Circuit to FIRRTL Circuit (CHIRRTL) conversion
  2. Conversion of Chisel Annotations to FIRRTL Annotations
  3. Generation of RunFirrtlTransformAnnotations

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