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Implement Discriminated Unions (#149)

* Implement Discriminated Unions

If:
- all the alternatives of a union are records
- they have at least one field in common, that is a literal in each
record with unique values

Then when checking the alternatives we can pick the correct one first,
and then perform the validation, thus producing more specific error
messages.

This is something already present in TypeScript itself https://www.typescriptlang.org/docs/handbook/advanced-types.html#discriminated-unions
Though this implementation would not work in all the cases covered by
TS, but its a start.

* Remove perf optimisation

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