Case-insensitive keyword matching

I'm trying to migrate a megaparsec parser to flatparse, but it uses case-insensitive keywords. It would be really convenient to have case-insensitive versions of string, char, and switch. char isn't so hard to do. The other two look considerably harder, since the number of branches grows exponentially if using the same trie technique. Adding the API anyway would be really helpful even if the result is slower than the case-sensitive variant.

Depending on how my initial trials go with flatparse, I may have cycles to work on this, if you're interested in a pull request.

Start by examining the existing case-sensitive string, char, and switch implementations and the trie technique they use. Define the case-insensitive API and matching behavior, then compare its implementation and performance with the case-sensitive variants; done means all three operations support case-insensitive keywords without breaking existing parsing.