Do semantic analysis on lambda's bodies

[emmatyping]

Lambdas were seen as dynamic functions when going to report errors. This
changes them to not be dynamic functions and report semantic analysis
errors in their bodies.
Fixes #4098

The relevant check for is_dynamic() is here:

mypy/mypy/semanal.py

Line 3790 in b240125

self.function_stack[-1].is_dynamic()):

[ilevkivskyi]

I think it is worth adding few tests for this.

[emmatyping]

D'oh it appears I forgot to check in the tests I wrote. Just pushed them.

[gvanrossum]

Nice, but it's introducing a new form of error. Here's an example:

from typing import Callable
def f() -> Callable[[], int]:
    a = lambda: b  # error: Name 'b' is not defined
    b = 42
    return a
assert f()() == 42

I wouldn't mind so much except I'm seeing this example in a few places in our existing codebase where fixing it would be somewhat awkward (the lambda is passed as a callback to something that must exist before some local variable can be set -- but the callback won't be called until after that's taken care of).

[ilevkivskyi]

I think with PEP 526 this can be safely fixed by b: int somewhere at the right scope. Otherwise # type: ignore here I think is still better than uncaught errors somewhere else. (There seem to be no middle ground here, either lambdas are considered dynamic and not typechecked, or they are always checked.)

[gvanrossum]

Fair enough, I'll try to work around these.