* Use `ByteString.foldr` instead of `(List.foldr . BS.unpack)`
* Avoid calling `chr` and its test that checks for invalid Unicode
codepoints: we stay in the ASCII range so we know we're ok
* Avoid calling `isPrint` (unsafe FFI call): we can check the ASCII
printable range directly
* Use bit operations (`unsafeShiftR`, `.&.`) instead of `div` and `mod`