Move char/unified conversion into precompiled Util module

Move the `.char_to_unified` and `.unified_to_char` fundamental conversions into their own module, and then precompile the results for all known Emoji character values.  This makes these conversions effectively zero-cost for all known Emoji values.  If nothing is matched, falls back to the actual conversion functions.

This adds a fair amount of code complexity, and because to precompile stuff you need any function it depends on to be in a different module, hence the nested Util modules here.

Speed comparison can be seen here, check out the Exmoji PR#1 column:
https://docs.google.com/spreadsheets/d/1T08I6dlyFNqqdtvQykNt43tdT85lJy4US2SqErcpZ7g/edit?usp=sharing

Most interesting benefit I see in the speed increases is the knock-on effects in making `.scan/1` even faster, so the amount of text that could be scanned by a single node is even higher.  That said, we were already pretty darn fast, so the code complexity (especially the potential to introduce ordering gotchas in compilation) might not be worth it here.