add oneOf

[azerum]

My try on generic function for use case #6

For the following structure:

const OneOf = sd.oneOf({
  a: sd.uint8,
  b: sd.array(sd.uint8, sd.uint8)
});

It will create Serdes<T> where T is:

type OneOf = {
  type: 'a';
  value: number;
} | { 
  type: 'b';
  value: number[];
}   

P.S. Thank you for such an elegant and nice to use library

[weisrc]

Thank you for contributing! Seems like it fails in automated tests. I will investigate why.