feat: Add verification that the RingSig contains a specific Ring within

[h5law]

Overview

Currently during the Verify method on the RingSig type we ensure the ring signature is valid. However, as long as the ring was valid on creation any signatures made with it indefinitely will be valid. I propose the following method to be added to ensure the RingSig has an up to date version of the ring. This is vital for our usecase where the ring is created via known data (available to all) and thus when verifying it we want to ensure the signer is using the latest copy of the ring (and they still have permission to be signing things)

func (ring *Ring) Equals(other *Ring) bool {
	for i, p := range ring.pubkeys {
		if eq := p.Equals(other.pubkeys[i]); !eq { return false }
	}
	bp, abp := ring.curve.BasePoint(), ring.curve.AltBasePoint()
	obp, oabp := other.curve.BasePoint(), other.curve.AltBasePoint()
	if eq := bp.Equals(obp); !eq { return false }
	if eq := abp.Equals(oabp); !eq { return false }
	return true
}

func (sig *RingSig) CheckRing(ring *Ring) bool {
	return sig.ring.Equals(ring)
}

[noot]

@h5law thanks for the issue, yeah this makes sense to add. I'd prefer something like:

func (ring *Ring) Equals(other *Ring) bool {
	// code here
}

func (sig *RingSig) Ring() *Ring {
	return sig.ring
}

so that the user can do sig.Ring().Equals(otherRing).