Implement ElemOf as an integer tag

[isovector]

This PR implements ElemOf as an (unsafe) newtype around Int, and then does some pattern-synonym trickery to re-expose the safe Here/There constructors.

In the real world, this is a significant improvement in polysemy's code generation. When building Galley.API.Public from wireapp/wire-server#1917, this change is:

- terms: 16,086
+ terms: 13,280
- types: 7,910,353
+ types: 6,465,139
- coercions: 13,919,222
+ coercions: 13,522,063
- ppr core size: 951956810
+ ppr core size: 949201550

Fixes #427