Create SplitMessage() to replace SeparateKeyAndData(...int)

Keep SeparateKeyAndData(_ int, _ int) for backwards compatibility
with go-mobile bindings.
Deprecate SeparateKeyAndData in favor of SplitMessage.

crypto/sessionkey_test.go

@@ -278,7 +278,7 @@ func TestMDCFailDecryption(t *testing.T) {
 		t.Fatal("Expected no error when unarmoring, got:", err)
 	}
 
-	split, err := pgpMessage.SeparateKeyAndData()
+	split, err := pgpMessage.SplitMessage()
 	if err != nil {
 		t.Fatal("Expected no error when splitting, got:", err)
 	}