Nested Digests

[curran]

Within a reactive function, it should be possible to invoke digest multiple times.

[curran]

Necessary for datavis-tech/reactive-vis#14

[curran]

I wonder if the current implementation supports multiple levels of nesting... I'm not sure it does. It wipes out the changed nodes before and after the topological sort and dataflow graph execution, but I wonder if it might be better to use a stack here, pushing and popping the context.

[curran]

Added a test for doubly nested digest. It does work in the current implementation. It appears that the computed topological sort serves as a kind of call stack, so I think digests can be nested arbitrarily deeply.