Implement specially-solved TagToEnum class

See #19915. Also implements #16989. Fixes #8318.

There are still several TODOs, and lots of missing documentation and tests. But it's coming along. Let's see what CI thinks so far.

TODOs:

• Finish handling ExoticEnums in GHC.Core.Opt.ConstantFold
• (Look into the example in Note [Ticks and mandatory eta expansion] in GHC.CoreToStg.Prep
• Add various test cases
• Add/update release notes and changelog entries
• Write up a CLC ticket
• Find/make a GHC ticket for DataToTag/TagToEnum instances in signature files