Remove dependency on AsRef trait for TextBuffer

[Kryptos-FR]

Summary
Remove the dependency on the AsRef<str> trait for TextBuffer.

Rationale
That dependency is not needed as it is trivial to implement fn as_str(&self) -> &str for both str and String which are the default implementations provided by egui and very likely the one that most user will ever need. Then TextEdit can just use as_str() in places where it was using as_ref() before, which in my opinion makes it more consistent with the trait definition.

In one of the project I am working on we did implement a custom TextBuffer for our struct Var. The forced dependency on AsRef<str> caused some pains because:

• our struct Var is a kind of container that can have all kind of underlying data type (not only string).
• in other places, it forced us to use the turbofish where it was not needed before:

// before implementing TextBuffer for Var
let headers: &mut Var = self.some_function();
let mut headers: Table = headers.as_ref().try_into()?; // here it uses the usual as_ref()

// after
let headers: &mut Var = self.some_function();
let mut headers: Table = <&mut Var as AsRef<Var>>::.as_ref(&headers).try_into()?; // here as_ref() was ambiguous

We anticipate that other occurrences will appear over time and we would like to avoid them.

Checklist

• It is a trivial change.
• I have run ./sh/check.sh.
  • There are errors but unrelated to this change.

[emilk]

Sounds good to me!