Pros: Ability to use Rust libraries from crates.io
Problems: Need to lock library versions and document their reproducible building, either via Cargo.lock or vendoring.
This allows us to remove a lot of #ifdef's. Either we want to use
a config.h or we don't. Since we'll want it for e.g. optional gsound
support later on let's have it.
This breaks autoconf. The only resulting binary is the squeekboard GUI. It still needs the autotools-built eekboard client in order to do anything useful. That one needs to be built using a different branch, making this a WIP.
