Add `s-less?' predicate.

This is a simple wrapper around the built-in `string-lessp'.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>
Updated documentation.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>
Merge pull request #24 from ruediger/lessp

Add `s-less?` predicate.