Support type aliases

[thangngoc89]

So I was trying to decode a result type like this:

type message = Belt.Result.t(list(parse_result), Error.t);

And I got this error:

The value t_encode can't be found in Belt.Result

With deriving plugins, I'd normally create a type alias like this:

[@decco]
type result('a, 'b) = Belt.Result.t('a, 'b) = | Ok('a) | Error('b);

But now the error is still the same.

[ryb73]

Thanks for the request. I haven't gotten a chance to look into this yet, but regarding the specific case you mentioned, support for Belt.Result.t was recently added in #19.