The first type represents a result of an unresolved member chain,
and the second type is its base type. This constraint acts almost
like `Equal` but also enforces following semantics:
- It's possible to infer a base from a result type by looking through
this constraint, but it's only solved when both types are bound.
- If base is a protocol metatype, this constraint becomes a conformance
check instead of an equality.
Currently the pattern is to collect the type variables and then unique
them. Instead of asking clients to do uniquing, let's just accept a set
as an argument.
