Hash table threshold is now float, not double

Hash table threshold is now float, not double

Change default from 0.8 to 0.8125 so it fits in float without
rounding glitches.
* doc/lispref/hash.texi (Creating Hash):
* doc/lispref/objects.texi (Hash Table Type):
* etc/NEWS:
Document change.
* src/fns.c (make_hash_table, maybe_resize_hash_table)
(Fmake_hash_table): Threshold is now float, not double.
Be consistent about how this is rounded.
* src/lisp.h (struct Lisp_Hash_Table.rehash_threshold):
Change back to float, now that the other code rounds consistently.
(DEFAULT_REHASH_THRESHOLD): Now float 0.8125 instead of double 0.8.