Improve hashing quality when configured --with-wide-int.

Improve hashing quality when configured --with-wide-int.

* fns.c (hash_string): New function, taken from sxhash_string.
Do not discard information about ASCII character case; this
discarding is no longer needed.
(sxhash-string): Use it.  Change sig to match it.  Caller changed.
* lisp.h: Declare it.
* lread.c (hash_string): Remove, since we now use fns.c's version.
The fns.c version returns a wider integer if --with-wide-int is
specified, so this should help the quality of the hashing a bit.