Reorganize whitespace stripping in _tokenize()
Reorganize whitespace stripping in _tokenize()

By consistently stripping trailing whitespace instead of stripping
initial whitespace in the string/operator case, we do less redundant
work and handle the '\n's at the end of lines better. Shaves a few % off
the _tokenize() runtime in cProfile and line_profiler.

ulfalizer/Kconfiglib,ulfalizer/Kconfiglib