Handle "size loops" when changing structure

[DrChainsaw]

Removing a vertex which happens to be the only SizeAbsorb vertex in a fork-path which is eventually joined by a SizeInvariant vertex is makes the graph invalid if there is a SizeStack with more than one input after the removed vertex.

Reason is that this results in the impossible situation where the output size of the SizeStack must be equal to its own output size (due to SizeInvariant forcing this on the path where the SizeAbsorb vertex was removed) plus a non-zero term (from the other inputs).

Possible strategies to deal with this are:
1. Detect the size-cycle and don't remove such vertices (probably easy). done in #35
2. Remove the whole path (might be hard).
3. Remove the vertex and connect the loose ends to some "nearby" vertices (another path maybe) for which the situation does not occur (head hurts).

[DrChainsaw]

Don't forget to add the above for edge creation and (most likely) removal.