Use hash_idx_t for storing hash indices

Use hash_idx_t for storing hash indices

Now hash_idx_t is a typedef for ptrdiff_t so there is no actual code
change, but this allows us to decouple the index width from the Lisp
word size.

* src/lisp.h (hash_idx_t): New typedef for ptrdiff_t.
(struct Lisp_Hash_Table): Use it for indices and sizes:
index, next, table_size, index_size, count and next_free.
All uses adapted.