Split repo into app and library code bases

[oubiwann]

https://users.rust-lang.org/t/what-is-the-idiomatic-way-to-manage-a-project-with-multiple-crates/6683

When this gets worked on, it should probably be in its own milestone, since this is a big (breaking) change.

Tasks

• [x] Move source code into new subdirs
• [x] Update source files for the new locations/library names
• [x] Ensure builds, tests, etc., continue to work
• [x] Get rucksack/README.md rendering

[oubiwann]

Related:

• 74

(Some tasks that were originally part of ticket #70 have been moved there.)

[oubiwann]

So far so good. Any issue that crop up with this, I'll open as separate bug tickets.