Make build output more useful for debugging

Turn on logging using the RUST_LOG environment, and clear all other
environment when running rust implementation.

Use 'readlink -f' to get the full canonical name of the source so that
the error messages from the compiler and tests are usable in Emacs.
Add vars module

Add a skeleton builder and state module with failing tests, and add a
vars module for loading the build variables that has some tests.

Also, first macro usage in rust -- whoot!