Equatability allows faster implementations for updating cached grapheme boundary state after a text mutation, because it enables quick detection of before/after state equality, without having to feed the recognizers until they produce a synchronized grapheme break.
The CustomStringConvertible conformance makes it orders of magnitude more pleasant to debug code that uses this.
Sendable is a baseline requirement for value types these days.
`Unicode._CharacterRecognizer` is a newly exported opaque type that
exposes the stdlib’s extended grapheme cluster breaking facility,
independent of `String`.
This essentially makes the underlying simple state machine public,
without exposing any of the (unstable) Unicode details.
The ability to perform grapheme breaking over, say, the scalars stored
in multiple `String` values can be extremely useful while building
custom text processing algorithms and data structures.
Ideally this would eventually become API, but before proposing this
to Swift Evolution, I’d like to prove the shape of the type in actual
use (and we’ll also need to find better names for its operations).
