Add a Cargo-based build system

This commit is the start of a series of commits which start to replace the
makefiles with a Cargo-based build system. The aim is not to remove the
makefiles entirely just yet but rather just replace the portions that invoke the
compiler to do the bootstrap. This commit specifically adds enough support to
perform the bootstrap (and all the cross compilation within) along with
generating documentation.

More commits will follow up in this series to actually wire up the makefiles to
call this build system, so stay tuned!