el_key - alpha version

[MartinKavik]

#354

 circle![
    el_key(&key),   // key implements ToString
    attrs![
        At::Cx => 15.0 + x,
        At::Cy => 15.0 + y,

[David-OConnor]

I like the API

[MartinKavik]

API is consistent with the el_ref but it should only require PartialEq instead of ToString.
I chose ToString because String implements Debug & Clone and we don't have to add another generic parameter to El and adapt the entire codebase to it.

[TatriX]

Should it be Display instead? Citing docs:

This trait is automatically implemented for any type which implements the Display trait. As such, ToString shouldn't be implemented directly: Display should be implemented instead, and you get the ToString implementation for free.

[MartinKavik]

Yeah, I know this advice. I don't implement ToString, I'm just using impl instead of generics here.
I think - if I understand that advice correctly - that ToString is basically something like a subset of Display. So I chose to write &impl ToString because it sounds to me a little bit more relaxed than Display. And I don't want to "display" the key anywhere, I just want to convert it to String so it feels also more descriptive to me.

[MartinKavik]

Closing in favor of #357 (that PR is based on this one)