unicorn 2.0.0 - mostly internal cleanups

Despite the version number, this release mostly features
internal cleanups for future versions of Rainbows!.  User
visible changes include reductions in CPU wakeups on idle sites
using high timeouts.

Barring possible portability issues due to the introduction of
the kgio library, this release should be ready for all to use.
However, 1.1.x (and possibly 1.0.x) will continue to be
maintained.  Unicorn 1.1.5 and 1.0.2 have also been released
with bugfixes found during development of 2.0.0.