Add a guix.scm for easy building on Guix.

[attila-lendvai]

This enables the following on Guix:

To build and install a local, possibly modified checkout: guix package -f guix.scm

To build it, but not install it: guix build -f guix.scm

To use as the basis for a shell with the development environment: guix shell

[melted]

Thanks!