Sync with SoLit signals

[duncsully]

I basically entirely reimplemented and rethought out signals in my SoLit repo. I believe they have feature parity, but would not be entirely backwards compatible with nubbins. e.g. I explicitly separated writable and computed signals and also separated out the update functionality into an update method instead of accepting a function to set React state setter style. I would likely need to update some of the FE library adapters. My goal eventually would be to have nubbins be my primary signals-based state management library and then add it as a dependency to SoLit.