Global variables

[bartvm]

See #4 (comment) and subsequent discussion. Related to #11.

Python's name bindings are mutable.

x = 1
def f():
    return x
x = 2
assert f() == 2

This can be handled by reparsing the function each time it is called, to ensure that the current values of globals are used. A more efficient way would be to only redo the parsing when the values have changed; to make globals explicit as parameters to a main function that the other functions are closed over; or to introduce environment objects.