general cleanup of the codefolding code

- const& where applicable
- const methods where applicable
- less copies where needed, foreach e.g. does an implicit copy already
- pass const& instead of pointers where applicable
- reuse existing code, esp. for the decrement/incrementBy1
- less copies by using foo& = bla[x];
- introduce temporaries instead of repetitive calls to bla[]
also makes the code nicer to read
- give saner name to some temporaries, instead of 'tempFoo'
- some more assertions to make sure our assumptions on non-empty lists holds
- probably more