Add chisel3.stage.ChiselStage

This adds ChiselStage, a reimplementation of chisel3.Driver as a
firrtl.options.Stage. This is simplistically described as a pipeline
of Phases.

Co-Authored-By: Schuyler Eldridge <schuyler.eldridge@ibm.com>
Co-Authored-By: chick <chick@qrhino.com>
Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>