Scope explicit edges to/validate against lifelines

[MattWindsor91]

Further to #90, we need to make sure that any to and from on ExplicitEdges can only pick up actors that are in the parent sequence's lifetime set. This won't be automatic, so I'll need to add this scoping restriction explicitly.

[MattWindsor91]

Actually, this is more intricate than I'd expect: not all ExplicitEdges have parent sequences. Instead, what I think we need is a validation that looks something like: any edge that appears in a sequence must have its resolved to and from appear in the lifelines.