Add --handover option to lower partially using SFC

Add a "handover" option that augments circt.stage.ChiselStage to lower
using a combination of the Scala FIRRTL Compiler (SFC) followed by
CIRCT.  The handover can be set at CHIRRTL (don't run the SFC), High
FIRRTL, Middle FIRRTL, Low FIRRTL, or Optimized Low FIRRTL.

Include tests of this behavior by looking at the
FirrtlCircuitAnnotation that exists before running CIRCT for different
handover values.

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