derivations: Introduce 'graft' record type.

* guix/derivations.scm (<graft>): New record type.
  (graft-derivation): Rename 'replacements' to 'grafts', and expect it
  to be a list of <graft> records.  Adjust accordingly.
* tests/derivations.scm ("graft-derivation"): Use 'graft' instead of
  pairs in argument to 'graft-derivation'.