Support non-ASCII identifiers

[lotem]

Support Unicode identifiers.

Refer to the similar language feature in Rust:
rust-lang/rfcs#2457

[lotem]

Hi John-John @udoprog,

The non_ascii_idents feature is now merged to stable Rust. It would be nice to have Unicode identifier names in Rune too.
So far the feature seems to work after a few changes. Please advice if there's more to do.

[udoprog]

Hi!

This looks great, thanks for the PR!

Please move the test into crates/rune/tests. They will be picked up automatically there.

A cargo feature flag is not appropriate since it transitively changes the behavior of anyone using Rune if one dependency activates it. In my view we should just adopt this for 0.14. If we do think it's an appropriate optional feature, add a an Options flag so that individual projects can decide themselves whether or not to activate it.

[lotem]

Thanks for the suggestions. I'll update the PR later.

[lotem]

I was worried about introducing a dependency for a rarely needed feature. Otherwise I don't think users will ever want to disable it since it's a compatible change. Also the performance report from unicode-ident seems to be less of a concern.

[udoprog]

The dependency is not a problem so please go ahead.

[udoprog]

Thank you!