Create specially-solved DataToTag class

See #20532 (closed). (And also see ghc-proposals#530 for my interest in that issue.)

See this Core Libraries Commitee issue on the API design for this MR

TODO:

• internal documentation (Note in GHC.Tc.Instance.Class, etc.)
• external documentation (changelog entry, etc.)
• code clean-up
• test cases
• Add "small" dataToTag primop? (left as future work)
• hide the underlying primop(s) (hidden from base only)
• figure out what to do for properly lev-poly data declarations