itz.Either Alias With Arrays

[zeluisping]

Is your feature request related to a problem? Please describe.
Having to write itz.Either(...) every time I have a union type is very annoying.

Describe the solution you'd like
Simply use arrays of validators as an alias to itz.Either, it looks way cleaner.

Describe alternatives you've considered
There really isn't a simpler alternative than this.

Additional context

Before

const foo = itz.A({
    foo: itz.Either(itz.String, itz.AsNumber, itz.Undefined),
});

After

const foo = itz.A({
    foo: [itz.String, itz.AsNumber, itz.Undefined],
});

This together with #4 would allow for cleaner structure definitions.