A few new useful type traits.

Yes, these are all available through e.g. Boost, and including them in
FTL might seem a bit like NIH or re-inventing the wheel. However,
having FTL depend on Boost is unacceptable--FTL is a stand-alone
library--and the traits are desirable.
Compile time check for Orderable instances.

(Default check uses traits to look for operators ==, <, >)
Minor fixes to type signatures.
Unit tests for ord module.