Add support for multi-file projects

[zsup]

[dbrgn]

This probably means "support for modules in separate files".

I did a first try a while ago:

https://github.com/dbrgn/particle-firmware/commit/b3d2daef986d1ee77bf86bc164e161f764911836

This works in theory, the only problem is that Make tries to build each .rs file with rustc separately (which fails for all rust files except the main one). If you invoke rustc manually it works, so it's only a build system issue.