$.exactUnion

[harrysolovay]

While an all-encompassing union codec (such as that described in #152) might be excessive, we may benefit from an $.exactUnion codec, which would be used to describe exact discriminated unions.

const $a = $.object(
  $.field("a", $.str),
  $.optionalField("b", $.never),
)

const $b = $.object(
  $.optionalField("a", $.never),
  $.field("b", $.u8),
)

const $c = $.exactUnion(
  $.variant("a", $a),
  $.variant("b", $b),
)

type C = $.Input<typeof $c>
// {
//   a: string
//   b?: never
// } | {
//   a?: never
//   b: number
// }

This would useful should we wish to describe Capi's config with a SCALE codec (which would allow us to validate the config). See paritytech/capi#924.