change `ITreeConstraint` into marker interface
move CLP(FD) work into it's own namespace

CLP(FD) now resides in it's own namespace with exception of a few
remaining complected bits. Updated tests and benchmarks. All tests pass.
fix some typos in bench.clj
remove typo
update CHANGES & README, bump project.clj
[maven-release-plugin] prepare release core.logic-0.8.0-rc2
[maven-release-plugin] prepare for next development iteration