py/objstr: Remove "make_qstr_if_not_already" arg from mp_obj_new_str.

This patch simplifies the str creation API to favour the common case of
creating a str object that is not forced to be interned.  To force
interning of a new str the new mp_obj_new_str_via_qstr function is added,
and should only be used if warranted.

Apart from simplifying the mp_obj_new_str function (and making it have the
same signature as mp_obj_new_bytes), this patch also reduces code size by a
bit (-16 bytes for bare-arm and roughly -40 bytes on the bare-metal archs).

extmod/modwebrepl.c

@@ -141,7 +141,7 @@ STATIC void handle_op(mp_obj_webrepl_t *self) {
     // Handle operations requiring opened file
 
     mp_obj_t open_args[2] = {
-        mp_obj_new_str(self->hdr.fname, strlen(self->hdr.fname), false),
+        mp_obj_new_str(self->hdr.fname, strlen(self->hdr.fname)),
         MP_OBJ_NEW_QSTR(MP_QSTR_rb)
     };