Set PairingOutput::default() = TargetField::one()

[rozbb]

Previously, PairingOutput<P>::default() was set to P::TargetField::zero(). This is clearly incorrect, since 0 is not in the multiplicative subgroup that PairingOutput represents. This concretely lead to an unexpected error in arkworks-rs/ripp#50.

This PR sets the default value to P::TargetField::one(). I don't think you have to treat this as a breaking change. It is very unlikely that someone was using this behavior successfully, since if you multiply (Add) this value with any other PairingOutput, you get 0.

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