TODO:

• Make Readable and Writable dyn compatible. Most of the complex logic is split out into extension traits which makes this PR breaking
• Implement map_mut. This requires two different closures to map the immutable and mutable borrows since closures cannot be generic over mutability
• Make BoxedReadable Copy
• Merge ReadOnlySignal and BoxedReadable (and keep ReadOnlySignal as a type alias)

Closes #3610