[Feature] Add Interface, ConformsTo for Separate Compilation (#3199)

Migrate work from seldridge/chisel-separable on separate compilation
into upstream Chisel.  This defines an Interface and a Conformance.  An
Interface is a Singleton Object that maps to a FIRRTL external module
with pre-defined IO.  A Conformance is a type-class whose
implementations define how a Chisel module aligns itself to the
Interface.  Used together, these enable separate compilation of a client
that uses an Interface from a component that implements the Interface.

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