Add macro for importing a type (ala import_exception)

[alex]

Currently import_exception makes it easy to import and use an exception type defined in Python from Rust. It'd be useful to have a generalized version of this that gave you a type.

Right now it can be accomplished by writing imperative py.import().getattr() statements, but these are quite verbose.

It'd be a big ergonomics win to have a builtin way to do this.

[davidhewitt]

I drafted such a thing a long time ago in https://github.com/davidhewitt/pyo3-pytypes - it's horribly broken though. We could try to revive it.

I'd prefer not put an API in pyo3 main repo until we figure out how it interacts with subinterpreters. #576 has details on what kinds of internals are a problem.

[alex]

Hmmm, ok. Sub-interpreters feels like a very deep mess.