Add Alt for decoder functions

[mlms13]

We currently have examples in tests where we use <|> from Result or Option to essentially fall through multiple decoders until we find one that works. This isn't ideal because it requires you to run each decoder, even if the first one succeeds.

Arguably, Decode.oneOf already solves this use case, but in the spirit of making decoders more composable (#23), we could add Decode.alt that looks something like this:

type stringOrInt = S(string) | I(int);
let decode =
  Decode.alt(
    Decode.map(v => S(v), Decode.string),
    Decode.map(v => I(v), Decode.intFromNumber),
  );

// Ok(I(3));
let decoded = decode(Js.Json.number(3.0));

• Since you'd be applying alt to the decode functions themselves (instead of the output option/result), we should be able to only run subsequent decoders if we haven't yet found a successful value
• We can probably rewrite oneOf to use this alt function?
• If possible, this is a good candidate to use the TriedMultiple error to track the failure from each decoder that has been attempted (Add a new ParseError constructor "TriedMultiple" #26)

[mlms13]

TriedMultiple is still coming, and I haven't yet written tests to prove that the alt implementation is lazy (it definitely is). Other than that, this is done.