Implement discriminated union schema

[lazamar]

Better way to specify schemas for discriminated unions than using oneOf.

const schema = discriminatedUnion([
  variant({ ty: 'one', a: string }),
  variant({ ty: 'two', b: number }),
]);

type Ty = Infer<typeof schema>;

/* inferred type:
type Ty = {
    readonly ty: "one";
    readonly a: string;
} | {
    readonly ty: "two";
    readonly b: number;
}
*/

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[tjschutte]

@codex review

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[EthanRBrown]

This is a definite improvement, thank you! Couple of minor notes, but approved