store: Raise error conditions upon protocol errors.

* guix/store.scm (&nix-error, &nix-protocol-error): New SRFI-35
  condition types.
  (process-stderr): Raise an error condition upon protocol errors
  instead of returning to the caller.  This allows the connection to be
  reused for further interactions.

Add (guix utils), with `bytevector->base32-string'.

* guix/utils.scm, tests/utils.scm: New files.

Use libchop for cryptographic hashes and related.

* guix/derivations.scm (sha256): Rewrite using libchop's
  `bytevector-hash'.
  (derivation-hash): Add docstring.

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.