Version 0.16 of Lily.

This adds a handful of neat things to the language. Built-in types are
now Titlecased, which I've quickly started to like. Option and Either
now come standard, and Option in particular is starting to become
very useful. I've used Option to make a lot of core apis not longer
raise an error.

The message for this tag is shorter than previous ones, because much
of what could be said here is instead in the RELEASES.md file.
