Speed up qstr loading by using the stack to store a temporary

string instead of the heap.

py/persistentcode.c

@@ -124,10 +124,9 @@ STATIC size_t read_uint(mp_reader_t *reader) {
 
 STATIC qstr load_qstr(mp_reader_t *reader) {
     size_t len = read_uint(reader);
-    char *str = m_new(char, len);
+    char str[len];
     read_bytes(reader, (byte*)str, len);
     qstr qst = qstr_from_strn(str, len);
-    m_del(char, str, len);
     return qst;
 }