Change hash range reduction from remainder to multiplication

Change hash range reduction from remainder to multiplication

This makes both lookups and rehashing cheaper.  The index vector size
is now always a power of 2.  The first table size is reduced to
6 (from 8), because index vectors would become excessively big
otherwise.

* src/lisp.h (struct Lisp_Hash_Table): Replace index_size with
index_bits.  All references adapted.
(hash_table_index_size): New accessor; use it where applicable.
* src/fns.c (hash_index_size): Replace with...
(compute_hash_index_bits): ...this new function, returning the log2 of the
index size.  All callers adapted.
(hash_index_index): Knuth multiplicative hashing instead of remainder.
(maybe_resize_hash_table): Reduce first table size from 8 to 6.