Graduate DataPayloadOr

[sffc]

We already use it in icu_datetime, and I want to use it in icu_locale_names. The design is good enough; I would like to graduate this. It's hard for us to change it anyway because it isn't behind an unstable feature.

The only real downside is that it's not obvious when to use this type instead of Option<DataPayload>. It exists to reduce stack sizes, full stop. It deviates from Rust style, but has convenient constructors and methods that make it feel like an Option<DataPayload>.

Changelog

icu_provider: Add type DataPayloadOr (note: it is already in the 1.5.x changelog)

[Manishearth]

I think this is fine, the API is pretty straightforward.

[sffc]

https://icu4x-pr-artifacts.storage.googleapis.com/gha/275bee8edbf527f7b8e7e1080da47c62edc1fa9e/rustdoc/icu_provider/struct.DataPayloadOr.html

@robertbastian Docs explaining why this type exists and what it does

[robertbastian]

A comment on the docs:

Please don't use the term "word". Rust has a concrete type for what you mean: usize. Word is a bad term because it sometimes doesn't mean the architecture-specific bit-width, but a fixed size data type (and it's also a linguistic term).