derivations: Avoid string-to-bytevector conversions.

derivations: Avoid string-to-bytevector conversions.

On Guile 2.2.1, this yields a 5% speedup on:

  guix build libreoffice xmonad certbot -n --no-substitutes --no-build-hook

* guix/derivations.scm (derivation->string): Rename to...
(derivation->bytevector): ... this.  Use 'open-bytevector-output-port'
instead of 'call-with-output-string'.
(derivation-hash): Remove string-to-bytevector conversion before
'sha256' call.
(build-expression->derivation): Use 'add-data-to-store' and an
bytevector port instead of a string port for the expression.