Signal traits

[jquesada2016]

This is intended to be a largish PR. The theme of this game is to make all core shared methods on signals into traits. I have added try_ variants to all of them. The Untracked* traits also need to be updated, but perhaps in a future PR, as this one is large enough as is.

[gbj]

Thanks. I just looked over this and it looks like reverting the changes to StoredValue is the only thing missing that I can see.

I am also thinking that signals could be a better module name than signal_prelude, so that someone can use leptos::signals::*;

I'm going to do some work on reorganizing the other re-exports now too.