[stdlib] Remove StringRef from python binding functions

[martinvuyk]

Remove StringRef from python binding functions.

CC: @ConnorGray I'd also like to deprecate .unsafe_cstr_ptr() in PR #3638

[ConnorGray]

Hey Martin, thanks for tackling this! 🙂

Seeing all the places here where you needed to add an O: ImmutableOrigin parameter made me want to re-evaluate our current API design for StringSlice, to see if we can't avoid having to add so many new parameters (in this PR, and in existing usage of StringSlice).

I'm going play around with a couple design ideas, namely variations on removing the // from StringSlice so that the is_mutable parameter is no longer infer-only so that the mutability can be affixed directly as something like StringSlice[Immutable].

This is something we had before the language added infer-only parameters, but IMO in the case of is_mutable parameters, making them unspecifiable has this slight usability downside.

I'll report back here once that lands, and then see how it affects this PR and move forward with it 🙂

[ConnorGray]

Question Why force the return value to be an immutable slice?

[martinvuyk]

Because otherwise we'd need to add the equivalent of Span.get_immutable() and do string_var.as_string_slice().get_immutable() which is way too verbose. And I figured that to keep the scope of this PR small it might be better to just force the API to return immutable slices. I'm fine with doing it either way, WDYT?