feat: add Equal method to compare keys

Closes #125

key.go

@@ -85,6 +85,9 @@ type PublicKey interface {
 	// `KeyType() == KeyTypeRSA2` would both have `BaseType() == KeyTypeRSA`.
 	BaseType() NID
 
+	// Equal compares the key with the passed in key.
+	Equal(key PublicKey) bool
+
 	evpPKey() *C.EVP_PKEY
 }
 
@@ -109,6 +112,10 @@ type pKey struct {
 
 func (key *pKey) evpPKey() *C.EVP_PKEY { return key.key }
 
+func (key *pKey) Equal(other PublicKey) bool {
+	return C.EVP_PKEY_cmp(key.key, other.evpPKey()) == 1
+}
+
 func (key *pKey) KeyType() NID {
 	return NID(C.EVP_PKEY_id(key.key))
 }

key_test.go

@@ -36,6 +36,10 @@ func TestMarshal(t *testing.T) {
 		t.Fatal(err)
 	}
 
+	if !key.Equal(key) {
+		t.Fatal("key not equal to itself")
+	}
+
 	privateBlock, _ := pem_pkg.Decode(keyBytes)
 	key, err = LoadPrivateKeyFromDER(privateBlock.Bytes)
 	if err != nil {