Made TreeFiniteDomainSolver incrimentally expand tree.

Bumped up the Scala version to v2.10.3.
Made Equal[VariableName] implicit a val.
Added FastSymbol to replace Symbol internal use.

The replacement benchmarks at about 3x faster for lookups and 2x
faster for instance creation.
Also bumped up library dependencies for Scalaz, ScalaTest, and
ScalaMock.  All of this warranted a minor version increase to
v0.5.2-SNAPSHOT.