Warn if generator is iterated over more than once

Somewhat related to #4707

This code currently does not give any warnings:

gen = (3 * x for x in range(10))  # revealed type: Generator[int, None, None]
for i in gen:
    print(i)

for i in gen:
    print(i)

and while this does run, it doesn't do what the programmer expects. Only the first iteration prints anything. The second iteration stops immediately because the generator is already exhausted.

This might be very difficult, but it would be nice if mypy could detect this.

Start with the generator example in the issue and investigate how mypy could identify that the same generator is iterated more than once. Done would be a warning for the second iteration without warning on valid generator use; the issue names no files or tests, so the relevant implementation and regression-test locations must first be identified.