Improve performance of ChiselEnum annotations (#2923) (#2927)

ChiselEnums check if they should create annotations every time an
instance of them is bound. Because so many annotations would be created,
they check to see if an equivalent annotation has already been added to
the annotations. Previously, this used a linear search of the
annotations, now it uses a HashSet.

(cherry picked from commit 96bde092

)

Co-authored-by: Jack Koenig <koenig@sifive.com>