partial rebase of PeterCrozier's enum work onto current master

partial rebase of PeterCrozier's enum work onto current master

I tried to keep only the enum-related changes, and minimize the diff. (The
original commit also had a lot of work done to get typedefs working, but yosys
has diverged quite a bit since the 2018-03-09 commit, with a new typedef
implementation.) I did not include the import related changes either.

Original commit:
"Initial implementation of enum, typedef, import.  Still a WIP."
https://github.com/PeterCrozier/yosys/commit/881833aa738e7404987646ea8076284e911fce3f