Add unit test for derivation parsing and output.

* guix/derivations.scm: Export `read-derivation' and `write-derivation'.

* srfi/srfi-64.scm, srfi/srfi-64.upstream.scm: New files, from libchop.
* tests/derivations.scm, tests/test.drv: New files.

Add `.gitignore'.

Split (guix) in (guix store) and (guix derivations).

* guix.scm: Move contents to other files.  Just aggregate these.
* guix/derivations.scm, guix/store.scm: New files.

Initial commit.