SignalExt::map_value_signal, signal_map::always

[MartinKavik]

• Add SignalExt::map_value_signal.

  • Inspired by SignalVecExt::map_signal.
  • New tests map_value_signal_const_signal and map_value_signal_const_map. Tested also manually in my app.
  • Not sure if we want to change it to SignalExt::map_signal or SignalExt::map_entry_signal and pass also Key or &Key into the callback.

• Add signal_map::always.

  • Inspired by signal_vec::always.

• Update CHANGELOG.md.

• Notes

  • cargo fmt (--all) was formatting too much code so I just tried to follow the code style manually.
  • I had to disable one linter in src/signal/mod.rs to compile the project (see the related TODO).
  • I've found a breaking change in previous commits. The code map_ref! .... => { ... return x ... } no longer works, it has to be replaced with the code like map_ref! .... => 'block: { ... break 'block x ... }.

[MartinKavik]

Thanks! I'm back from vacation now, so I should be a bit more active.

Don't hurry, I hope you enjoyed your vacation :slightly_smiling_face:

Note: I've added one new commit with your suggested change and one related question in a comment.