Speed up sxhash-equal-including-properties

Speed up sxhash-equal-including-properties

This function now no longer conses at all.  Previously, it constructed
a list structure of all string intervals for the sole purpose of
hashing.

* src/fns.c (hash_interval): New.
(Fsxhash_equal_including_properties):
Use it instead of collect_interval.

(cherry picked from commit 3869944bb4f9434e0c49063a291ed8a0a33cba50)