* guix/store.scm (%store-prefix): New parameter.
(store-path?, derivation-path?): New procedures.
* guix/derivations.scm (write-derivation): Pass SOURCES through
`object->string'.
(compressed-hash, store-path, output-path, derivation): New
procedures.
* tests/derivations.scm (%store): New global variable.
("derivation with no inputs"): New test.
* 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.