Python runtime name resolution

[bartvm]

See #4 (comment) and subsequent discussion.

Python's runtime name resolution means that the name binding of a closed variable can be changed.

def f():
    x = 3
    def g():
        return x
    x = 4
    def h():
        return x
    return g, h

g, h = f()
assert g() == h()

We should either disallow this, specialize g multiple times, or perform name resolution at runtime instead of at compile-time.

[bartvm]

Relevant quote from the Python documentation:

On the other hand, the actual search for names is done dynamically, at run time — however, the language definition is evolving towards static name resolution, at “compile” time, so don’t rely on dynamic name resolution! (In fact, local variables are already determined statically.)