Fix glitches in recent hash table changes

Fix glitches in recent hash table changes

* src/fns.c (Fmake_hash_table): Simplify the machine code slightly
by using 0 rather than -1.
* src/lisp.h (struct Lisp_Hash_Table.pure): Now bool rather
than a bitfield, for speed (the bitfield did not save space).
(struct Lisp_Hash_Table.rehash_threshold): Now double rather than
float, since the float caused unwanted rounding errors, e.g.,
(hash-table-rehash-threshold (make-hash-table)) yielded
0.800000011920929 instead of the correct 0.8.