[Feature request] Ergonomic borrow structures with static lifetime for baked data providers

[csmulhern]

As far as I can tell, you can only get TypeBorrowed<'static> versions of structs without unsafe when using compile_data.

E.g. when using ComposingNormalizer, all interesting methods are on ComposingNormalizerBorrowed. ComposingNormalizer::new_nfc returns a ComposingNormalizerBorrowed<'static> for use with compile_data.

When using a baked data provider, the only alternative I see is to do e.g.

let normalizer = icu::normalizer::ComposingNormalizer::try_new_nfc_unstable(&BakedDataProvider).unwrap();

If implementing a struct that owns a ComposingNormalizer, you're then forced to call ComposingNormalizer ::as_borrowed whenever you need to actually use the normalizer.

Alternatively, you can use unsafe to create a ComposingNormalizerBorrowed<'static>, although it's unclear if this is actually safe or not:

let normalizer = icu::normalizer::ComposingNormalizer::try_new_nfc_unstable(&BakedDataProvider).unwrap();
let normalizer: icu::normalizer::ComposingNormalizerBorrowed<'static> =
            unsafe { std::mem::transmute(normalizer.as_borrowed()) };

Am I doing something wrong? Is there a more ergonomic API that could be added here (or clarification added in the docs whether extending the lifetime here is safe or not)?