Improve type mismatch error message by providing hints when the solution might be guessable by the compiler

[autodidaddict]

As a concrete example, when someone mutates a field inside a fun, the error message doesn't suggest that the solution might be to convert it into a ref fun to accommodate the mutation. Instead you often see errors that complain about x is not a subtype of y or that it was looking for this->x and it saw x. These are technically valid error messages, but if the compiler can detect that it's actually just a simple mutability issue, it could provide a hint.

[sheerluck]

I want to work on it ^_^