Migrate OnchainEvent serialization to be MaybeReadable.

This adds a new TLV-based enum serialization macro entitled
`impl_writeable_tlv_based_enum_upgradable`. As the name implies,
the new macro allows us to ignore odd-numbered variant entries.
Because the new macro implements only `MaybeReadable` and not
`Readable`, it is not applicable in many contexts, here only being
added for the two `OnchainEvent` structs.