Add NODISCARD to all {Decode,Parse}[...](...) functions returning bool. Sort includes.

src/attributes.h

@@ -0,0 +1,22 @@
+// Copyright (c) 2009-2010 Satoshi Nakamoto
+// Copyright (c) 2009-2018 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_ATTRIBUTES_H
+#define BITCOIN_ATTRIBUTES_H
+
+#if defined(__has_cpp_attribute)
+#  if __has_cpp_attribute(nodiscard)
+#    define NODISCARD [[nodiscard]]
+#  endif
+#endif
+#ifndef NODISCARD
+#  if defined(_MSC_VER) && _MSC_VER >= 1700
+#    define NODISCARD _Check_return_
+#  else
+#    define NODISCARD __attribute__((warn_unused_result))
+#  endif
+#endif
+
+#endif // BITCOIN_ATTRIBUTES_H