[assembler] put ApplyOn back to functors
[assembler.functors] updated typedefs
[assembler] moved walker_wrapper.hh -> walker.hh
[assembler.wrapper] put into internal namespace

There should be no need for users to touch them.
[assembler.gridwalker] updated includes
[assembler] moved local_wrapper to wrapper
[assembler] moved gridwalker, functors and wrapper to dune-stuff