Move GetStackTrace and Demangle to wpiutil, add Windows support (#1819)

For Windows, import the StackWalker source (https://github.com/JochenKalmbach/StackWalker)
plus PR 35 in that repo, with a few simplifications to StackWalker.h.

wpiutil/src/main/native/windows/Demangle.cpp

@@ -0,0 +1,30 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2008-2019 FIRST. All Rights Reserved.                        */
+/* Open Source Software - may be modified and shared by FRC teams. The code   */
+/* must be accompanied by the FIRST BSD license file in the root directory of */
+/* the project.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#include "wpi/Demangle.h"
+
+#include <windows.h>  // NOLINT(build/include_order)
+
+#include <dbghelp.h>
+
+#include "wpi/mutex.h"
+
+#pragma comment(lib, "Dbghelp.lib")
+
+namespace wpi {
+
+std::string Demangle(char const* mangledSymbol) {
+  static wpi::mutex m;
+  std::scoped_lock lock(m);
+  char buffer[256];
+  DWORD sz = UnDecorateSymbolName(mangledSymbol, buffer, sizeof(buffer),
+                                  UNDNAME_COMPLETE);
+  if (sz == 0) return mangledSymbol;
+  return std::string(buffer, sz);
+}
+
+}  // namespace wpi