Reconsider InferredStatementContet.childCopyType storage

The work in YANGTOOLS-1253 achieved its basic goals trimming down object sizes in most deployements while creating some free space.

One remaining item is InferredStatementContet.childCopyType, which is using a reference for something which is only 2 bits of semantics. We have 4 bits still available in StatementContextBase.copyHistory, hence it might make sense to eliminate the field and reconstruct the object on demand.

Doing so will translate into further savings of 8/8/0/0 bytes – which may be interesting, but does not really change anything in most deployments.