Don't inherit SingleIdent from Ident
We don't rely on that inheritance anywhere, and it allows more efficient storage of both as well as avoids the extra allocation for the storage in a list in SingleIdent.
This matters as this is on the hot path for folder loading and message list population.
There's more that can be done here, such as not disassembling the identifier into its parts, but that can be done separately without affecting ABI.