Faster hash table growth, starting at zero size

Faster hash table growth, starting at zero size

The algorithms no longer use the rehash_threshold and rehash_size
float constants, but vary depending on size.  In particular, the table
now grows faster, especially from smaller sizes.

The default size is now 0, starting empty, which effectively postpones
allocation until the first insertion (unless make-hash-table was
called with a positive :size); this is a clear gain as long as the
table remains empty.  The first inserted item will use an initial size
of 8 because most tables are small.

* src/fns.c (std_rehash_size, std_rehash_threshold): Remove.
(hash_index_size): Integer-only computation.
(maybe_resize_hash_table): Grow more aggressively.
(Fhash_table_rehash_size, Fhash_table_rehash_threshold):
Use the constants directly.
* src/lisp.h (DEFAULT_HASH_SIZE): New value.