Implement :keyword support

This implements keywords, ":" prefixed symbols that are able to look
themselves up in a collection. They're internally stored as strings that
start with "\ufdd0".

This fixes #22.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
tests: Test that keywords do not clash with normal strings

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
Merge branch 'f/hy.models.keyword' of git://github.com/algernon/hy into pr-101