Consider removing `KeyPair` class in favor of a record `typedef KeyPair<T,S> = ({T privateKey, S publicKey});`

[jonasfj]

We could do:

typedef KeyPair<T,S> = ({T privateKey, S publicKey});

Instead of having a KeyPair class, it's not like we need the class for anything, all it does is create a type that doesn't have to exist.

It would be a cleaner API t use a record.

Given that there is no public constructor for KeyPair and the fact that it is final; I'm not even sure it's really a breaking change.

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Making this change is probably easy. But I think we should land it along with a breaking version bump, so 0.5.x -> 0.6.0 or something like that.