Move harfbuzz-traits implementations into component crates

[robertbastian]

icu_properties already does this for unicode-bidi. The current icu_harfbuzz crate is written for use with the harfbuzz crate, however there are other Rust HarfBuzz implementations that don't need the code in that crate.

[sffc]

Seems good to eliminate a leaf crate. Can we avoid growing the boilerplate in examples/cargo/harfbuzz?

[robertbastian]

the boilerplate in the example is additional example code for using buffer providers, as well as the ZST box optimization that not relevant to all HarfBuzz implementations (the harfbuzz crate needs it because it needs to box over FFI).

[Manishearth]

Overall in favor of this!

@hsivonen wrote this, we should probably wait for him to check in

[Manishearth]

Would still prefer that @hsivonen sign off on this being removed.

[hsivonen]

Moving the trait implementations seems OK.

It would be good to publish a tombstone release of icu_harfbuzz (with [badges] maintenance = { status = "deprecated" }) to crates.io when when icu_properties and icu_normalizer with this change reach crates.io.

[Manishearth]

That's a good point. @robertbastian can you do that after merging this?

(my r+ stands, feel free to merge)

[robertbastian]

I filed #7210 because we can only do this after 2.2.