#5 Migrate to typed racket

Based on some quick testing, using typing adds overhead to the execution time unless the program is compiled. While the overall runtime is generally "fast enough" (~0.4s without typing, ~1s with typing), it would be nice if this program were snappy. If this issue is addressed and the program is migrated to Typed Racket, it would probably be preferable to encourage users to compile the program (`raco exe` or `raco compile`) before running it, to improve responsiveness.