py/qstr: Add qstr_from_strn_static() helper function.

Allows an interned string to reference static/ROM data, instead of
allocating it on the GC heap.

Signed-off-by: Damien George <damien@micropython.org>

py/qstr.h

@@ -101,6 +101,9 @@ qstr qstr_find_strn(const char *str, size_t str_len); // returns MP_QSTRnull if
 
 qstr qstr_from_str(const char *str);
 qstr qstr_from_strn(const char *str, size_t len);
+#if MICROPY_VFS_ROM
+qstr qstr_from_strn_static(const char *str, size_t len);
+#endif
 
 mp_uint_t qstr_hash(qstr q);
 const char *qstr_str(qstr q);