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.h

@@ -732,6 +732,12 @@ extern const struct _mp_obj_exception_t mp_const_GeneratorExit_obj;
 
 // General API for objects
 
+// Helper versions of m_new_obj when you need to immediately set base.type.
+// Implementing this as a call rather than inline saves 8 bytes per usage.
+#define mp_obj_malloc(struct_type, obj_type) ((struct_type *)mp_obj_malloc_helper(sizeof(struct_type), obj_type))
+#define mp_obj_malloc_var(struct_type, var_type, var_num, obj_type) ((struct_type *)mp_obj_malloc_helper(sizeof(struct_type) + sizeof(var_type) * (var_num), obj_type))
+void *mp_obj_malloc_helper(size_t num_bytes, const mp_obj_type_t *type);
+
 // These macros are derived from more primitive ones and are used to
 // check for more specific object types.
 // Note: these are kept as macros because inline functions sometimes use much