Promote ChiselEnum user APIs from experimental (backport #2929) (#2931)

* Promote ChiselEnum user APIs from experimental (#2929)

They are commonly used and if we were ever to change them, we will need
to go through a deprecation process anyway.

Note that the EnumAnnotations remain in chisel3.experimental because,
like all Chisel annotation support, they are slated to be deprecated and
removed.

(cherry picked from commit 424e9446

)

# Conflicts:
#	docs/src/cookbooks/verilog-vs-chisel.md
#	docs/src/explanations/chisel-enum.md

* Maintain binary compatbility, use aliases in chisel3 package

* Deprecate the actual classes in chisel3.experimental
* Also fix backport conflicts

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