Signal finder

[pvh]

A signals-lite based implementation of the new approach to find(). I think this is now on a pretty promising path it:

• can return a value synchronously if it's available (check the current state of the signal)
• supports many listeners
• handles unassisted progress (whoops on the generator)

The signals implementation is and is intended to be quite minimal but it probably needs a few more minor features. Eliminating manual deregistration would be one goal, and progressing in "epochs" to avoid glitches in the matrix would be the other. Cycle detection was a good idea. I may yet cave and adopt / vendor something small into the library but this isn't meant to be a broadly adopted signals library, just a small thing that we can adapt into the many other frameworks and libraries in the ecosystem.

Some notes: as of this writing there are still about a third of the repo tests failing. If this direction continues to work I'll probably move the find() logic into a standalone file so that it can be more easily inspected / tested. I'd also like to try updating DocHandle to emit one of these signals (at least optionally) as the document evolves.

I'd also like to reduce the duplication between dochandle and the find code in terms of state tracking as part of the cleanup but that might mean some adjustment of the current soon-to-be-deprecated sync protocol and I'm avoiding that can o' worms.