guix: Move narinfo code from substitute script to module.

This separation between the code for dealing with narinfos from the code doing
that for a purpose should make things clearer, and better support components
other that the substitute script in using this code.

This is just moving the code around, no code should have been significantly
changed.

* guix/scripts/substitute.scm (<narinfo>): Move record type to (guix narinfo).
(fields->alist, narinfo-hash-algorithm+value, narinfo-hash->sha256,
narinfo-signature->canonical-sexp, narinfo-maker, read-narinfo,
narinfo-sha256, valid-narinfo?, write-narinfo, narinfo->string,
string->narinfo, equivalent-narinfo?, supported-compression?,
compresses-better?, narinfo-best-uri): Move procedures to (guix narinfo).
(%compression-methods): Move variable to (guix narinfo).
* guix/narinfo.scm: New file.
* Makefile.am (MODULES): Add it.
* po/guix/POTFILES.in: Add 'guix/narinfo.scm'.