feat: add ring equality check and expose ring on ringsig

[h5law]

Summary

This PR addresses issue #13 by adding the following methods:

func (r *Ring) Equals(other *Ring) bool
func (rs *RingSig) Ring() *Ring

These allow for the RingSig to check whether it contains a specific ring using the following flow:

if eq := sig.Ring().Equals(otherRing); !eq {
	panic("ring sig doesnt contain otherRing")
}