* [TypeResolver][TypeChecker] Add support for structural opaque result types
* [TypeResolver][TypeChecker] Clean up changes that add structural opaque result types
Conformance constraints could be transferred through conversions,
but that would also require checking implicit conversions
such as optional and pointer promotions for conformance is the
type itself doesn't conform, for that let's add a special constraint
`TransitivelyConformsTo`.
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.
