py: Rework mp_convert_member_lookup to properly handle built-ins.

This commit fixes lookups of class members to make it so that built-in
functions that are used as methods/functions of a class work correctly.

The mp_convert_member_lookup() function is pretty much completely changed
by this commit, but for the most part it's just reorganised and the
indenting changed.  The functional changes are:

- staticmethod and classmethod checks moved to later in the if-logic,
  because they are less common and so should be checked after the more
  common cases.

- The explicit mp_obj_is_type(member, &mp_type_type) check is removed
  because it's now subsumed by other, more general tests in this function.

- MP_TYPE_FLAG_BINDS_SELF and MP_TYPE_FLAG_BUILTIN_FUN type flags added to
  make the checks in this function much simpler (now they just test this
  bit in type->flags).

- An extra check is made for mp_obj_is_instance_type(type) to fix lookup of
  built-in functions.

Fixes #1326 and #6198.

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

py/objgenerator.c

@@ -73,6 +73,7 @@ STATIC mp_obj_t gen_wrap_call(mp_obj_t self_in, size_t n_args, size_t n_kw, cons
 
 const mp_obj_type_t mp_type_gen_wrap = {
     { &mp_type_type },
+    .flags = MP_TYPE_FLAG_BINDS_SELF,
     .name = MP_QSTR_generator,
     .call = gen_wrap_call,
     .unary_op = mp_generic_unary_op,
@@ -126,6 +127,7 @@ STATIC mp_obj_t native_gen_wrap_call(mp_obj_t self_in, size_t n_args, size_t n_k
 
 const mp_obj_type_t mp_type_native_gen_wrap = {
     { &mp_type_type },
+    .flags = MP_TYPE_FLAG_BINDS_SELF,
     .name = MP_QSTR_generator,
     .call = native_gen_wrap_call,
     .unary_op = mp_generic_unary_op,