py/objfun: Implement function.__code__ and function constructor.

This allows retrieving the code object of a function using
`function.__code__`, and then reconstructing a function from a code object
using `FunctionType(code_object)`.

This feature is controlled by `MICROPY_PY_FUNCTION_ATTRS_CODE` and is
enabled at the full-features level.

Signed-off-by: Damien George <damien@micropython.org>

tests/basics/fun_code_micropython.py

@@ -0,0 +1,19 @@
+# Test MicroPython-specific restrictions of function.__code__ attribute.
+
+try:
+    (lambda: 0).__code__
+except AttributeError:
+    print("SKIP")
+    raise SystemExit
+
+
+def f_with_children():
+    def g():
+        pass
+
+
+# Can't access __code__ when function has children.
+try:
+    f_with_children.__code__
+except AttributeError:
+    print("AttributeError")