Add helper methods to ChiselStage for Driver migration

This adds three new methods to ChiselStage to replace deprecated
methods in the Driver for converting a Chisel circuit to a string:
  - emitChirrtl
  - emitFirrtl
  - emitVerilog

This also adds a ChiselStage companion object that lets you generated
a Chisel Circuit or a FIRRTL Circuit from a Chisel module:
  - elaborate
  - convert

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

squash! Add string emission helper methods to ChiselStage