Import Bech32 C++ reference code & tests

This includes a reformatted version of the Bech32 reference code
(see https://github.com/sipa/bech32/tree/master/ref/c%2B%2B), with
extra documentation.

src/bech32.h

@@ -0,0 +1,25 @@
+// Copyright (c) 2017 Pieter Wuille
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+// Bech32 is a string encoding format used in newer address types.
+// The output consists of a human-readable part (alphanumeric), a
+// separator character (1), and a base32 data section, the last
+// 6 characters of which are a checksum.
+//
+// For more information, see BIP 173.
+
+#include <stdint.h>
+#include <string>
+#include <vector>
+
+namespace bech32
+{
+
+/** Encode a Bech32 string. Returns the empty string in case of failure. */
+std::string Encode(const std::string& hrp, const std::vector<uint8_t>& values);
+
+/** Decode a Bech32 string. Returns (hrp, data). Empty hrp means failure. */
+std::pair<std::string, std::vector<uint8_t>> Decode(const std::string& str);
+
+} // namespace bech32