Add Meta to all SPDX Elements and parser support

Elements: Package, Checksum, File, CreationInfo, VerificationCode,
Review, All Licences.

A lot of changes in parsing licences sets in RDF due to initially
using []AnyLicence to represent a not-yet-known LicenceSet without
meta. When meta was added, this trick does not work anymore thus I
had to refactor this logic.

Signed-off-by: Vlad Velici <vlad.velici@gmail.com>