Use ptrdiff_t for hash table indexes.

Use ptrdiff_t for hash table indexes.

* category.c (hash_get_category_set):
* ccl.c (ccl_driver):
* charset.h (struct charset.hash_index, CHECK_CHARSET_GET_ID):
* coding.c (coding_system_charset_list, detect_coding_system):
* coding.h (struct coding_system.id):
* composite.c (get_composition_id, gstring_lookup_cache):
* fns.c (hash_lookup, hash_put, Fgethash, Fputhash):
* image.c (xpm_get_color_table_h):
* lisp.h (hash_lookup, hash_put):
* minibuf.c (Ftest_completion):
Use ptrdiff_t for hash table indexes, not int (which is too
narrow, on 64-bit hosts) or EMACS_INT (which is too wide, on
32-bit --with-wide-int hosts).