[Ur] New release

[Adam Chlipala]

Available as usual at: http://www.impredicative.com/ur/

CHANGELOG for this release:

- Extend 'where con' to allow descending into submodule structure
- Type inference improvements
- Treat transactional FFI functions as effectful by default,
   so that 'effectful'/'benignEffectful' .urp directives aren't necessary
- Interpret 'table' signature items more flexibly,
   automatically adding (Ur) constraints to support
   a kind of subtyping over (SQL) constraint sets
- New header file scheme to support FFI code in either of C or C++
- New command-line arguments: -ccompiler, -print-ccompiler
- More HTML tags: <dd>, <dl>, <dt>, <em>, and <strong>
- Add 'rel' attribute to <a>
- New type synonym 'xhead'
- Refactored the development Mercurial repository to remove 
autogenerated files
- More integration with Autotools distribution functionality
- Improvements to documentation and compiler error messages
- Bug fixes and optimization improvements