Moves compare operators from TDictionary to TSymbol.

Issue #20
Renames TSymbol::less to TSymbol::TCompareFunctor.

Issue: #20
Merge remote-tracking branch 'origin/fix/20' into develop

* origin/fix/20:
  Renames TSymbol::less to TSymbol::TCompareFunctor.
  Moves compare operators from TDictionary to TSymbol.