Created a bare .ins and .dtx file

The package itself still has to be written of course.
Use `\DeclareDocumentCommand` from `xparse`

Also fixed the `.dtx` syntax a little
Fixed indentation of `macrocode` in `stacks.dtx`
Fixed a silly copy paste mistake
Implemented a `xkeyval` approach to options
Changed `concept.tex` to `test.tex`

Rudimentary test suite for what is implemented so far. Which is not much.