[mypyc] Avoid cyclic reference in nested functions

[JukkaL]

Mypyc used to always put nested functions into the environment object, which results
in cyclic references, since the function object contains a reference to the environment.

Now we only do this if the body of a nested function refers to a nested function (e.g.
due to a recursive call). This means that in the majority of cases we can avoid the
cyclic reference.

This speeds up self check by an impressive 7%. I'm not sure exactly why the impact
is so big, but spending less time in the cyclic garbage collector is probably a big part.

[ilevkivskyi]

Looks great! Just one suggestion for a test.

[ilevkivskyi]

I think it may be worth testing mutually recursive functions, something involving:

def f():
    def g():
        h()
    def h():
        g()
    return g, h

or are we already testing this?

[JukkaL]

Yeah, we already have at least the test case testNestedFunctions2 in mypyc/test-data/run-functions.test (mutual_recursion).