py/obj.h: For obj reprs A,B,C use void* explicitly for mp_obj_t typedef.

The machine_ptr_t type is long obsolete as the type of mp_obj_t is now
defined by the object representation, ie by MICROPY_OBJ_REPR.  So just use
void* explicitly for the typedef of mp_obj_t.

If a port wants to use something different then they should define a new
object representation.

py/obj.h

@@ -31,15 +31,15 @@
 #include "py/qstr.h"
 #include "py/mpprint.h"
 
-// All Micro Python objects are at least this type
-// The bit-size must be at least pointer size
-
+// This is the definition of the opaque MicroPython object type.
+// All concrete objects have an encoding within this type and the
+// particular encoding is specified by MICROPY_OBJ_REPR.
 #if MICROPY_OBJ_REPR == MICROPY_OBJ_REPR_D
 typedef uint64_t mp_obj_t;
 typedef uint64_t mp_const_obj_t;
 #else
-typedef machine_ptr_t mp_obj_t;
-typedef machine_const_ptr_t mp_const_obj_t;
+typedef void *mp_obj_t;
+typedef const void *mp_const_obj_t;
 #endif
 
 // This mp_obj_type_t struct is a concrete MicroPython object which holds info