Hide alphabet internals

[reynir]

Base64.alphabet leaks the internal emap array. This PR makes Base64.alphabet return a string instead. The following snippet shows how to break encode |> decode using `default_alphabet.

let a = Base64.(alphabet default_alphabet) in
let () = Array.iteri (fun i _ -> a.(i) <- int_of_char 'A') a in
Base64.(decode_exn (encode_string "Hello, World!"))

This is obviously a breaking change since the interface is altered. As an alternative, I fixed the problem by calling Array.copy. The code is available in my branch hide-internals2 (with a test).

I think it makes the most sense to have Base64.alphabet return a string. Base64.make_alphabet takes a string, and it's annoying for the user that the alphabet characters are returned as an int array.

[dinosaure]

Thanks, I force-pushed an ocamlformat pass, it will be integrated into the next release.

PS: sorry for the delay.