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/unix/StackTrace.cpp

@@ -0,0 +1,37 @@
+/*----------------------------------------------------------------------------*/
+/* 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/StackTrace.h"
+
+#include <execinfo.h>
+
+#include "wpi/Demangle.h"
+#include "wpi/SmallString.h"
+#include "wpi/raw_ostream.h"
+
+namespace wpi {
+
+std::string GetStackTrace(int offset) {
+  void* stackTrace[128];
+  int stackSize = backtrace(stackTrace, 128);
+  char** mangledSymbols = backtrace_symbols(stackTrace, stackSize);
+  wpi::SmallString<1024> buf;
+  wpi::raw_svector_ostream trace(buf);
+
+  for (int i = offset; i < stackSize; i++) {
+    // Only print recursive functions once in a row.
+    if (i == 0 || stackTrace[i] != stackTrace[i - 1]) {
+      trace << "\tat " << Demangle(mangledSymbols[i]) << "\n";
+    }
+  }
+
+  std::free(mangledSymbols);
+
+  return trace.str();
+}
+
+}  // namespace wpi