Remove `pass' from the language.

This also fixes a bug in the pass optimize missing branch where the code is
something like: [stmt, [], stmt]; in such case we want to filter out [], so
if we end up with [] we can optimize it. This fix is needed otherwise (do)
inside (do) are not properly optimized.

Signed-off-by: Julien Danjou <julien@danjou.info>
Merge branch 'jd/remove-pass' of git://github.com/jd/hy into pr-122