This commit adds support to the `marshal` module to be able to dump
functions that have child functions. For example:
import marshal
def f():
def child():
return 1
return child
marshal.dumps(f.__code__)
It also covers the case of marshalling functions that use list
comprehensions, because a list comprehension uses a child function.
This is made possible by the newly enhanced
`mp_raw_code_save_fun_to_bytes()` that can now handle nested functions.
Unmarshalling via `marshal.loads()` already supports nested functions
because it uses the standard `mp_raw_code_load_mem()` function which is
used to import mpy files (and hence can handle all possibilities).
Signed-off-by: Damien George <damien@micropython.org>
