[mypyc] Support @deprecated on functions and methods

[rheard]

Fixes mypyc/mypyc#1243.

The runtime @deprecated decorator (PEP 702, from warnings or typing_extensions) stores the message in the __deprecated__ attribute of the function that it wraps. Compiled functions don't support setting attributes, so a module that uses the decorator on a function or a method fails at import:

AttributeError: 'builtin_function_or_method' object has no attribute '__deprecated__'

With this PR mypyc no longer calls the decorator. The compiled function issues the warning itself when it's called. A method isn't a decorated method for mypyc any more, so it's still called natively, and these work as for any other method: property getters and setters, class and static methods, and overriding a native method (or being overridden by one). The implementation of an overloaded function works too. A generator or a coroutine warns when it's created, like with the runtime decorator.

Differences from interpreted Python:

• The function has no __deprecated__ attribute. This now also applies to nested and async functions,
  where the runtime decorator happened to work. I did this to keep the behavior uniform, but I can leave
  those on the runtime path if that's preferred. It would seem adding support for attributes on compiled methods like __deprecated__ might be a bigger change than even this large PR.