* lib/hashkey-string.h: New file.
* lib/hashkey-string.c: New file, based on lib/clean-temp-simple.c.
* modules/hashkey-string: New file.
* lib/clean-temp-simple.c: Include hashkey-string.h. Don't include
<limits.h>.
(clean_temp_string_equals, clean_temp_string_hash): Remove functions.
(SIZE_BITS): Remove macro.
(register_temporary_file): Use hashkey_string_equals and
hashkey_string_hash.
* modules/clean-temp-simple (Depends-on): Add hashkey-string.
