Type inference should dynamically resolve `typing.Self`

[eb8680]

We'd like to be able to use typing.Self to annotate return types of method Operations. From the typing standard library documentation and the official typing specification, Self is equivalent to a type variable in the following sense, so it should be straightforward to fold into our existing machinery for handling type variables in internals.unification:

class Foo:
  def return_self[Self: "Foo"](self: Self) -> Self:
    ...
    return self