Calculating Sqrts and fixed points works!
Most tests work except the combinators
Shifted the Propagator to a method argument rather than a class argument, so the Var and Rxs themselves are not beholden to a specific Propagator, and multiple propagators can be used on the same dataflow graph
everything passes
Added concurrency tests to enforce behavior when a signal is run multiple times in parallel. Almost all tests pass
large refactoring to isolate the spin-retry logic into SpinlockSignal
Big refactoring of unit tests. Generally they work, but some of them are failing non-deterministically, which will need to be tracked down.

- nested web page example works
- removed `debounce` from the readme, since it's not working
- separated unit tests which need asynchrony from those needing concurrency/parallelism
Merge & Cleanup. Most stuff passes most of the time!