Runtime: support multiple inputs

[josephjclark]

The runtime needs to be able to support multiple inputs to a node.

This doesn't need to do anything particularly clever - if multiple edges lead to a node, that node gets called multiple times. We just need to ensure the correct input state is used - but I think we've already solved that.

We need to remove the validation that enforces that the workflow must be a tree.

We currently have loop detection - I suggest we keep that in place.

I am hoping that because this isn't technically hard, and we've got solid and simple execution rules, that this won't be more than day's work.