Convert integers to and from hexadecimal strings in Elm.
-- Convert a hexdecimal string such as "abc94f" to a decimal integer.
Hex.fromString "a5" == Ok 165
Hex.fromString "hat" == Err "invalid hexadecimal string"
-- Convert a decimal integer to a hexdecimal string such as `"abc94f"`.
Hex.toString 165 == "a5"