shash: New "smap" functions for working with string-to-string maps.

shash: New "smap" functions for working with string-to-string maps.

An shash always has string keys and sometimes it hash string values as
well.  Usually the general-purpose shash functions are fine for working
with string-to-string maps, but this commit introduces a few more
specialized functions that only work with string-to-string maps.  It's not
clear yet to me whether this should actually be a new data structure, so
for now the new functions just work on shashes.

This commit also converts one user of shash_destroy() to use smap_destroy().
This is the only existing user of these functions that I spotted as a
trivial conversion candidate while grepping.

These new functions will see more use in the following commit.

Reviewed by Justin Pettit.