all: Use new mp_obj_new_str_from_cstr() function.

Use new function mp_obj_new_str_from_cstr() where appropriate.  It
simplifies the code, and makes it smaller too.

Signed-off-by: Jon Foster <jon@jon-foster.co.uk>

extmod/modwebrepl.c

@@ -146,7 +146,7 @@ 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)),
+        mp_obj_new_str_from_cstr(self->hdr.fname),
         MP_OBJ_NEW_QSTR(MP_QSTR_rb)
     };