* Fix automerge-c tests on mac
* Generate significantly smaller automerge-c builds
This cuts the size of libautomerge_core.a from 25Mb to 1.6Mb on macOS
and 53Mb to 2.7Mb on Linux.
As a side-effect of setting codegen-units = 1 for all release builds the
optimized wasm files are also 100kb smaller.
The assert_doc and assert_obj macros in automerge/tests::helpers are
useful for writing tests for any application working with automerge
documents. Typically however, you only want these utilities in tests so
rather than packaging them in the main `automerge` crate move them to a
new crate (in the spirit of `tokio_test`)
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
