* 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.