py/obj: Introduce mp_obj_malloc macro to allocate, and set object type.

This is to replace the following:

    mp_foo_obj_t *self = m_new_obj(mp_foo_obj_t);
    self->base.type = &mp_type_foo;

with:

    mp_foo_obj_t *self = mp_obj_malloc(mp_foo_obj_t, &mp_type_foo);

Calling the function is less code than inlining setting the type
everywhere, adds up to ~100 bytes on PYBV11.

It also helps to avoid an easy mistake of forgetting to set the type.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

py/obj.c

@@ -37,6 +37,13 @@
 #include "py/stackctrl.h"
 #include "py/stream.h" // for mp_obj_print
 
+// Allocates an object and also sets type, for mp_obj_malloc{,_var} macros.
+void *mp_obj_malloc_helper(size_t num_bytes, const mp_obj_type_t *type) {
+    mp_obj_base_t *base = (mp_obj_base_t *)m_malloc(num_bytes);
+    base->type = type;
+    return base;
+}
+
 const mp_obj_type_t *MICROPY_WRAP_MP_OBJ_GET_TYPE(mp_obj_get_type)(mp_const_obj_t o_in) {
     #if MICROPY_OBJ_IMMEDIATE_OBJS && MICROPY_OBJ_REPR == MICROPY_OBJ_REPR_A