* 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/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.
