SiblingMut does lots of cloning and Portgraph-reconstruction

[acl-cqc]

SiblingMut added in #522 does not store a PortGraph. So, when you call say SiblingMut::neighbours....we first build a (SiblingGraph and therefore its) PortGraph. Then we use that to build an iterator. Then, because the PortGraph will disappear before SiblingMut::neighbours returns, we then copy everything out of that iterator.

If we could return the PortGraph along with the iterator, that'd avoid one pass of copying, although we'd still end up building a PortGraph every time. Even this seems tricky though - I've tried various things with ouroboros but never quite got there.