rust: Split project 4 into two projects

[brson]

After #231 it has two basic goals: introduce multithreading, then remove locks from readers.

The project has twice the words of any other. It should be two different projects.

There's another question of the benchmarking section, which already carries a note about how complex it is. There is advanced criterion subject matter that I want to cover there, but it doesn't have to be done as written. As part of splitting the projects up, do something with the benchmark section. I am worried though that removing the benchmarking section would make project 4 too small, but worth trying and seeing. Maybe just delete the whole thing, and file an issue to re-add the criterion content, with a link to the commit the deleted the old text.

The advanced parallelism section of project 4 today is very exposition heavy, but ultimately is one big-bang "make it lock-free" step. During a rewrite I would split it into: split readers and writers into two, each with their own lock; remove the lock from the reader.