[Ur] New library repositories

[Adam Chlipala]

I've taken some of the code I've been working on and divided it into two 
(for now) libraries that should be of general interest.  They are 
available as public Mercurial repositories.

- Metaprogramming: http://hg.impredicative.com/meta
- GUIs: http://hg.impredicative.com/gui

This code requires the latest Ur/Web compiler from its public repo, 
which I've just updated today.