Make better use of fixnum range in sxhash etc

Make better use of fixnum range in sxhash etc

Recent hash table changes reduced the range of sxhash, sxhash-eq etc
to [0,2**32) on platforms with 62-bit fixnums.  This change makes them
use the full fixnum range again.  Hash table hashing is unaffected.

* src/fns.c (sxhash_eq, sxhash_eql): New.
(hash_hash_to_fixnum): Replace with...
(reduce_emacs_uint_to_fixnum): ...this.
(hashfn_eq, hashfn_eql, Fsxhash_eq, Fsxhash_eql, Fsxhash_equal)
(Fsxhash_equal_including_properties): Use the new functions.

(cherry picked from commit 50201e03b9c4133296dbd10e6c7ebd5dc2a62d50)