Add --dont-fold option to disable folding prim ops (#2040)

This adds a --dont-fold options (backed by a DisableFold annotation)
that lets a user specify primitive operations which should never be
folded. This feature lets a user disable certain folds which may be
allowable in FIRRTL (or by any sane synthesis tool), but due to inane
Verilog language design causes formal equivalence tools to fail due to
the fold.

Add a test that a user can disable `a / a -> 1` with a
DisableFold(PrimOps.Div) annotation.

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