Add WriteHDKeypath function and move *HDKeypath to util/bip32.{h,cpp}

Creates new files util/bip32.h and util/bip32.cpp for containing
BIP 32 stuff.
Moves FormatKeyPath from descriptor.cpp to util/bip32.
Adds a wrapper around it to prepent the 'm' for when just the
BIP 32 style keypath is needed.

src/util/bip32.cpp

@@ -0,0 +1,66 @@
+// 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.
+
+#include <sstream>
+#include <stdio.h>
+#include <tinyformat.h>
+#include <util/bip32.h>
+#include <util/strencodings.h>
+
+
+bool ParseHDKeypath(const std::string& keypath_str, std::vector<uint32_t>& keypath)
+{
+    std::stringstream ss(keypath_str);
+    std::string item;
+    bool first = true;
+    while (std::getline(ss, item, '/')) {
+        if (item.compare("m") == 0) {
+            if (first) {
+                first = false;
+                continue;
+            }
+            return false;
+        }
+        // Finds whether it is hardened
+        uint32_t path = 0;
+        size_t pos = item.find("'");
+        if (pos != std::string::npos) {
+            // The hardened tick can only be in the last index of the string
+            if (pos != item.size() - 1) {
+                return false;
+            }
+            path |= 0x80000000;
+            item = item.substr(0, item.size() - 1); // Drop the last character which is the hardened tick
+        }
+
+        // Ensure this is only numbers
+        if (item.find_first_not_of( "0123456789" ) != std::string::npos) {
+            return false;
+        }
+        uint32_t number;
+        if (!ParseUInt32(item, &number)) {
+            return false;
+        }
+        path |= number;
+
+        keypath.push_back(path);
+        first = false;
+    }
+    return true;
+}
+
+std::string FormatHDKeypath(const std::vector<uint32_t>& path)
+{
+    std::string ret;
+    for (auto i : path) {
+        ret += strprintf("/%i", (i << 1) >> 1);
+        if (i >> 31) ret += '\'';
+    }
+    return ret;
+}
+
+std::string WriteHDKeypath(const std::vector<uint32_t>& keypath)
+{
+    return "m" + FormatHDKeypath(keypath);
+}