bulletproofs: add new empty module

src/modules/bulletproofs/main_impl.h

@@ -0,0 +1,12 @@
+/**********************************************************************
+ * Copyright (c) 2020 Andrew Poelstra                                 *
+ * Distributed under the MIT software license, see the accompanying   *
+ * file COPYING or http://www.opensource.org/licenses/mit-license.php.*
+ **********************************************************************/
+
+#ifndef _SECP256K1_MODULE_BULLETPROOFS_MAIN_
+#define _SECP256K1_MODULE_BULLETPROOFS_MAIN_
+
+/* TODO */
+
+#endif