py/obj: Add static safety checks to mp_obj_is_type().

Commit d96cfd13e3 introduced a regression by breaking existing
users of mp_obj_is_type(.., &mp_obj_bool).  This function (and associated
helpers like mp_obj_is_int()) have some specific nuances, and mistakes like
this one can happen again.

This commit adds mp_obj_is_exact_type() which behaves like the the old
mp_obj_is_type().  The new mp_obj_is_type() has the same prototype but it
attempts to statically assert that it's not called with types which should
be checked using mp_obj_is_type().  If called with any of these types: int,
str, bool, NoneType - it will cause a compilation error.  Additional
checked types (e.g function types) can be added in the future.

Existing users of mp_obj_is_type() with the now "invalid" types, were
translated to use mp_obj_is_exact_type().

The use of MP_STATIC_ASSERT() is not bulletproof - usually GCC (and other
compilers) can't statically check conditions that are only known during
link-time (like variables' addresses comparison).  However, in this case,
GCC is able to statically detect these conditions, probably because it's
the exact same object - `&mp_type_int == &mp_type_int` is detected.
Misuses of this function with runtime-chosen types (e.g:
`mp_obj_type_t *x = ...; mp_obj_is_type(..., x);` won't be detected.  MSC
is unable to detect this, so we use MP_STATIC_ASSERT_NOT_MSC().

Compiling with this commit and without the fix for d96cfd13e3 shows
that it detects the problem.

Signed-off-by: Yonatan Goldschmidt <yon.goldschmidt@gmail.com>

py/objstr.c

@@ -2144,7 +2144,7 @@ STATIC NORETURN void bad_implicit_conversion(mp_obj_t self_in) {
 qstr mp_obj_str_get_qstr(mp_obj_t self_in) {
     if (mp_obj_is_qstr(self_in)) {
         return MP_OBJ_QSTR_VALUE(self_in);
-    } else if (mp_obj_is_type(self_in, &mp_type_str)) {
+    } else if (mp_obj_is_exact_type(self_in, &mp_type_str)) {
         mp_obj_str_t *self = MP_OBJ_TO_PTR(self_in);
         return qstr_from_strn((char *)self->data, self->len);
     } else {