Associated constant assignment versus equality (#2173)

Split the `where A == B` constraint in two: `where .A = B` produces a new
constraint from an old one, where the value of `.A` in the new constraint is
known and eagerly rewritten to `B`, and `where A == B`, which does not cause
`A` and `B` to be considered as identical by language rules but does permit
implicit (no-op) conversion between them.
 
This aims to provide an efficiently-computable and human-understandable type
equality rule, with type canonicalization and therefore transitive type
equality, without sacrificing too much in the way of ergonomics and without
sacrificing determinism, while still providing the full power of a general type
constraint system in a less ergonomic form.

Co-authored-by: josh11b <[email protected]>