Replace unic

[chris-ha458]

As mentioned before, i prepared a PR that replaces unic from the codebase.
a non exhaustive list of reasons to change :

• it is not being developed or maintained anymore (in fact most if not all of the developers are responsible for icu4x atm)
• it's unicode support is old. (10?)

Through this PR, now Unicode 15 is supported.

I haven't seen any regressions from --bin performance or cargo bench.
Since icu4x is actively being developed I expect that there would be more features including name or unicode ranges implementations or speedups. Unfortunately there is no speedup observed at the moment, but icu4x is actively tracking and performance regressions and striving to improve it as well.

Also I hope to make further idiomatic changes especially around the md module upon this platform.

I know this is a big change, but hopefully it will lead to easier development and reduced hardcoding down the road.

Take your time reviewing it and feel free to ask any questions.