Bundle icu

This bundles the latest `icu4c` (54.1) to help prevent load errors when icu is upgraded on the system.

I've held off on this for a _while_ because:
* it makes the gem ~24MB
* it pushes compile time up to ~2min from a couple of seconds

Other than that, I think it'll be nice to be able to have more control over what `icu4c` version we're linking against and hopefully prevent these load errors from happening ever again.

Possible additions to this before shipping
- [ ] support `dir_config` so users can still link against a system `icu4c`

@josh @scottjg @rtomayko @jakeboxer what do you guys think?