Python yield_loop macro

run.py

import macropy.activate
import test

test.py

from yield_loop import macros, yield_loop
from macropy.tracing import macros, show_expanded

with show_expanded:
    def f():
        with yield_loop(i, some_aysnc_iterator()):
            print(i)

yield_loop.py

from macropy.core.macros import *
from macropy.core.quotes import macros, q, ast, name

macros = Macros()

@macros.block
def yield_loop(tree, gen_sym, args, **kw):
    # We can't use 'target' because of https://github.com/lihaoyi/macropy/issues/64.
    target, args = args
    iter = gen_sym()
    with q as new_tree:
        name[iter] = ast[args]
        while (yield name[iter].fetch_next()):
            ast[target] = name[iter].next_object
    # Is there some form similar to ast[tree] that can be used in the above
    # quasiquoted block? ast[] seems to want expressions, not statements.
    new_tree[-1].body += tree
    return new_tree