Separate code from data and other stuff

Rationale:

- To be able to exclude "other stuff" when searching in all files for code
    - Exclusion by file type is not enough if there are many non-R files -- they are looked at anyway (at least by RStudio), and this slows down the search
- I find myself constantly switching between `lib` and `src`, sometimes `munge`. As it is now, there is a lot of "other stuff" between these precious folders. Having them closer together would simplify things.

Candidates: `lib`, `munge`, `src`. Did I miss anything?

Requires ~~#84~~ #90.

- [x] Internal: Store files in tar archives also verbatim, to track changes
- [x] Script to create TAR archives (and also closes #91)
- [x] Change structure internally
- [ ] Migration code (including version bump)
- [ ] Double-check website, e.g. there is a screenshot that shows the old folder structure