Allow decoding via an explicit version-number field.

[lortabac]

Currently the decoding functions like fromJsonAnyVersion work by trying to decode at each version until the first one succeeds. This is convenient as a default strategy because of its simplicity. However a better solution would be to rely on an explicit versionNumber field and decode to the target version directly. The implementation will probably require the SV singleton type (currently only on the singletons branch).