Add asmap utility which queries a mapping

The scripts for creating a compact IP->ASN mapping are here:
https://github.com/sipa/asmap

Co-authored-by: Pieter Wuille <pieter.wuille@gmail.com>

src/util/asmap.h

@@ -0,0 +1,10 @@
+// Copyright (c) 2019 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_UTIL_ASMAP_H
+#define BITCOIN_UTIL_ASMAP_H
+
+uint32_t Interpret(const std::vector<bool> &asmap, const std::vector<bool> &ip);
+
+#endif // BITCOIN_UTIL_ASMAP_H