Add better comparison with icu

[robertbastian]

No description provided.

[BurntSushi]

Thank you! This is very helpful.

I updated this branch to tweak the code example a bit, and made it pass cargo test --doc (since COMPARE.md is included in the rustdoc).

I think the biggest change is probably removing the as casts in favor of u8::try_from(...).unwrap().

I am hoping to do a jiff-icu crate, following the discussion at #50, at some point to make the conversions a bit simpler, unless y'all get to it first.

[robertbastian]

LGTM