We have several functions in `ty_extensions` for testing our constraint
set implementation. This PR refactors those functions so that they are
all methods of the `ConstraintSet` class, rather than being standalone
top-level functions. 🎩 to @sharkdp for pointing out that
`KnownBoundMethod` gives us what we need to implement that!
Markdown files within the mdtest/ subdirectory are tests of type inference and type checking;
executed by the tests/mdtest.rs integration test.
See crates/ty_test/README.md for documentation of this test format.