asserts: add snap-size and remove snap-build from snap-revision
iirc, the snap-revision assertion was original tightly bound to the snap-build assertion by a combination of the snap-revision's header fields, one of which was called snap-build (really the digest of that assertion).
snap-revision's primary key was changed some time ago to uncouple it from the snap-build assertion. At that time, snap-revision's snap-build header became redundant and, more importantly, incorrect.
snap-size was added to snap-revision to give the assertion the same level of proof for the snap as snap-build has.