Expose initializer for KeyPair

[Reiszecke]

I needed to quickly compare implementations with preset keys. Not sure how useful that is for others, your decision whether to merge.

[samueltangz]

Is the constructKeyPair function suffice for your use case? We can derive the public key from the secret key with this function.

[Reiszecke]

Now that's a weird one, I'm getting different public keys in different implementations which most likely should not be happening :x

I guess I should investigate this first, will update once I know more, thank you for your time mate