String hashing improvements (spread and performance)

String hashing improvements (spread and performance)

Fix gaps in hashing coverage in the middle and end of even fairly short
strings.  E.g., `outline-1`, `outline-2` etc all hashed to the exact
same value but with the patch, there are no collisions among the ~160000
symbols in the Emacs tree.

This change also improves average hashing speed by using fewer mixing
operations.

* src/fns.c (hash_string):
Use unit stride for fairly short strings, while retaining the cap of 8
samples for long ones.

Always hash the last word to ensure that the end of the string is
covered.  For strings shorter than a word, use fewer loads and a single
reduction step.