* 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.
* 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.