Sanitize-concat-fim

[cmwilhelm]

Single-function rust scripts for development work on learn2code.
Executable via cargo from shell with local files, per desired
workflow. Tests were written in Python just to expedite delivery.
There is also a lot of duplicate code around file IO that I imagine
will go away once d2tk is a thing.

The real files to pay attention to are concat.rs and fim.rs. The
sanitizing stuff is just @soldni 's code unmodified.

This is meant to function as a pipeline, where each component is optional.
However, here is the intended order of operation:

1. (optionally) sanitize
2. (optionally) concat
3. (optionally) FIM