Provide a C++ interface to the collator

[hsivonen]

Currently, the collator isn't exposed to C++ via Diplomat. It should be.

Apart from the constructor, the Collator object has three public methods in Rust:

• compare() (takes guaranteed-valid UTF-8)
• compare_utf8() (takes potentially-invalid UTF-8)
• compare_utf16() (takes potentially-invalid UTF-16)

I expect these to be exposed to C++ as follows:

• C++ compare() taking std::u16string_view: Rust compare_utf16()
• C++ compare() taking std::string_view: Rust compare_utf8()
• C++ compare_unsafe() taking std::string_view: Rust compare() with documentation note that the call is UB unless the C++ caller has ensured UTF-8 validity.

(Once we support C++20, there should be std::u8string_view overloads for the same things that have std::string_view versions.)

In Rust, these return Ordering. It's a bit unclear if they should return int with values -1, 0, 1 or something fancier in C++.

[hsivonen]

C++ compare() taking std::string_view: Rust compare_utf8()

To get Diplomat to use std::string_view in C++, the argument type of the method on the #[diplomat::opaque] ICU4XCollator should be &str, which should immediately get as_bytes() called on it.

C++ compare() taking std::u16string_view: Rust compare_utf16()

This requires more Diplomat configurability.

[echeran]

Fixed by #2498. That covers adding Collator FFI via Diplomat. The remaining details in the description of this issue -- about customizing the APIs produced in C++ -- are a part of the plan outlined in #2520.