Should we expose experimental modules at the metacrate level

[robertbastian]

Instead of icu::experimental::tranliterator, it could be icu::transliterator. Experimental code would still live in the icu_experimental crate, the metacrate would just do pub use icu_experimental::* instead of pub use icu_experimental as experimental. I think this would make the metacrate's root docs page nicer, as it would show more of ICU4X's functionality, even if some of it is experimental. It would also keep paths stable during stabilisation.

[sffc]

The crate icu is auto-upgraded with semver, but things in icu::experimental can change at will. I think it is important to emphasize in the module name that the code is unstable.