Add new_typeinfo to public plugin API for both SemanticAnalyzer and TypeChecker

[mkurnikov]

In django-stubs, it's used in a lot of places to generate one-off classes.

For semanal, there's https://github.com/python/mypy/blob/master/mypy/semanal.py#L3029, which used for the special casing of NamedTuple/TypedDict/others, which basically plugins in the core.

For type checking, there's no equivalent, but similar code is used here https://github.com/python/mypy/blob/master/mypy/checker.py#L3659

It would be also nice to be able to specify more than one base class.

[msullivan]

Adding something to the plugin API for semantic analyzers is definitely in scope, if you want to submit a PR.

Typechecking is a little less clear. I think that callable hack might have a bad interaction with the daemon that just doesn't come up much, but if we generate more classes at typecheck time it would.