Replace embedded Groups with type assertions

These were always nil and likely unintentional (I assume it started out
as a way to mock unimplemented interface methods?). It doesn't matter
much because these suites all implement `kyber.Group`, but any changes
to `kyber.Group` would have added panicing methods to these suites
without any type errors.

pairing/bls12381/circl/adapter.go

@@ -13,9 +13,10 @@ import (
 // used as a point from the group G1.
 type SuiteBLS12381 struct {
 	Suite
-	kyber.Group
 }
 
+var _ kyber.Group = (*SuiteBLS12381)(nil)
+
 // NewSuiteBLS12381 makes a new BN256 suite
 func NewSuiteBLS12381() *SuiteBLS12381 {
 	return &SuiteBLS12381{}

pairing/bls12381/kilic/adapter.go

@@ -13,9 +13,10 @@ import (
 // used as a point from the group G1.
 type SuiteBLS12381 struct {
 	Suite
-	kyber.Group
 }
 
+var _ kyber.Group = (*SuiteBLS12381)(nil)
+
 // NewSuiteBLS12381 makes a new BN256 suite
 func NewSuiteBLS12381() *SuiteBLS12381 {
 	return &SuiteBLS12381{}

pairing/bn254/adapter.go

@@ -13,9 +13,10 @@ import (
 // used as a point from the group G1.
 type SuiteBn254 struct {
 	*Suite
-	kyber.Group
 }
 
+var _ kyber.Group = (*SuiteBn254)(nil)
+
 // NewSuiteBn254 makes a new BN254 suite
 func NewSuiteBn254() *SuiteBn254 {
 	return &SuiteBn254{

pairing/bn256/adapter.go

@@ -13,9 +13,10 @@ import (
 // used as a point from the group G1.
 type SuiteBn256 struct {
 	*Suite
-	kyber.Group
 }
 
+var _ kyber.Group = (*SuiteBn256)(nil)
+
 // NewSuiteBn256 makes a new BN256 suite
 func NewSuiteBn256() *SuiteBn256 {
 	return &SuiteBn256{