py/objstr: Split mp_obj_str_from_vstr into bytes/str versions.

Previously the desired output type was specified.  Now make the type part
of the function name.  Because this function is used in a few places this
saves code size due to smaller call-site.

This makes `mp_obj_new_str_type_from_vstr` a private function of objstr.c
(which is almost the only place where the output type isn't a compile-time
constant).

This saves ~140 bytes on PYBV11.

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

ports/rp2/moduos.c

@@ -35,6 +35,6 @@ STATIC mp_obj_t mp_uos_urandom(mp_obj_t num) {
     for (int i = 0; i < n; i++) {
         vstr.buf[i] = rosc_random_u8(8);
     }
-    return mp_obj_new_str_from_vstr(&mp_type_bytes, &vstr);
+    return mp_obj_new_bytes_from_vstr(&vstr);
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_1(mp_uos_urandom_obj, mp_uos_urandom);