graph: allow indeterminate iterator length

Created by: kortschak

Please take a look.

I have decided to use a panic in the slicer functions rather than returning nil to avoid silent failure from incorrect use.

The next step for this is to go through the functions that we have an document those that are able to work on indeterminate iterators. Then fix those that could but currently can not.

The intention with the semantics around the magnitude of the length return value are that for disk-backed graph the size can be known, but it may not be materialisabe. This is useful information, just not information that can be used directly by most of the analytical functions.

Updates #627.