Add -X mhigh compiler for minimal high form (#2268) (#2269)

Add a compiler/emitter that can target minimal high form.  This will
produce output that only has CHIRRTL constructs removed.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>
(cherry picked from commit 3ea95e72

)

# Conflicts:
#	src/test/scala/firrtlTests/stage/FirrtlMainSpec.scala

Co-authored-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>