Common reactive interface

[ealmloff]

Specific Demand

It would be nice to provide a common way to read different reactive signal-like values. The common interface should support Signals, Fermi atoms, reactive collections (a signal that acts like Signal<Vec<Signal>>>) and mapped signals

Implement Suggestion

1. We signal something like Box<dyn Readable + Writable> and ReadOnlySignal something like Box<dyn Readable>
   a) This introduces extra overhead by boxing everything
   b) You can easily implement IntoSignal<T> for different values like T, and `
2. We could expose the traits Readable and Writable and let components take impl Readable + Writable or impl Readable
   a) It should be lower overhead than boxing everything
   b) You could try to use specialization to implement Readable for T and specialize it on Signal, MappedSignals, and Fermi atoms
   c) This could introduce a lot of generics which could be an ergonomic issue depending on how well impl trait is supported within components

[ealmloff]

Readable and Writable traits were merged in #1791