finer sorting of include's (C before C++)

- increased number of subdirectories of Core/
- moved files between subdirectories to minimize dependences
  - use inline function definitions if and only if fit into one line AND do not involve external dependences
- systematic use of forward class declarations to make .h files independent of other .h files
- forward class declarations only as separate commands, no longer as part of function or variable declarations
- unified code layout in passing
  - prefer 'T* t' and 'T& t' over 'T *t' and 'T &t' in  function or variable declarations

Move 'class' from variable declaration, function return types, and function arguments to separate statements.

In particularly, derived sample visitor classes now are explicitly "using" ISampleVisitor::visit.