Modernize LoKiCore
- do not explicitly disable default constructors which would not be generated implicitly anyway (also the dictionary generation seems to be happy with this!)
- make single-argument constructors 'explicit' (with a few exceptions!)
- (re-)implement FunctorFromFunctor assignment in terms of copy-and-swap, update swap implementation
- prefer STL algorithms over loops
- remove redundant explicit virtual d'tor implementations
- ... and prefer implicitly generated copy c'tor (which is now generated because the explicit d'tor is no longer there)