After some discussion with PVH I realise that the repo structure in the
last reorg was very rust-centric. In an attempt to put each language on
a level footing move the rust code and project files into ./rust
For larger rust projects it's common to put all rust code in a directory
called `crates`. This helps in general by reducing the number of
directories in the top level but it's particularly helpful for us
because some directories _do not_ contain Rust code. In particular
`automerge-js`. Move rust code into `/crates` to make the repo easier
to navigate.
The test `CMakeLists.txt` brings in cmocka but we don't actually need to
build the tests to get the docs. This just makes the cmake docs script
tell cmake not to build docs.
