Nonlocal statement support

[BPYap]

Nonlocal is used in some built-in module (e.g. tasks.py in asyncio module). Support for nonlocal statement should enable a number of files in KNOWN_PROBLEM_MODULES to compile.

Refer to GSOC week 10/11 log for implementation details.

New features added in this PR:

• nonlocal is now supported in function closure and generator closure. e.g:

def func():
    a = 'a from outer'
    def nested_func():
        nonlocal a
        print(a)  # will print 'a from outer'
        a = 'a from inner'
        print(a)  # will print 'a from inner'

    nested_func()
    print(a)  # will print 'a from inner'

func()

• Generator is now able to read variables from enclosing scopes. e.g:

def func():
    a = 'a from outer'
    def gen():
        print(a)  # will print 'a from outer'
        yield

    next(gen())

func()

[BPYap]

@freakboy3742, passing #locals as storage space for closure variables seems to work well. To my surprise it even fixes the test_recursive_in_function case 😄

Well, some part of the code may still be further enhanced to be more efficient.
For now, only function and generator support closure/nonlocal. I'm thinking to add closure support for class and method at later time and in separate PR as they are not as important in getting asyncio to work.

[freakboy3742]

👍 It took a while, but we finally got there! This looks a lot cleaner - excellent work!