[wpiutil] Add MessagePack utility functions (#4448)

Also add mpack to srcDirs.

wpiutil/src/main/native/cpp/MessagePack.cpp

@@ -0,0 +1,45 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+#include "wpi/MessagePack.h"
+
+using namespace mpack;
+
+mpack_error_t mpack::mpack_expect_str(mpack_reader_t* reader, std::string* out,
+                                      uint32_t maxLen) {
+  uint32_t count = mpack_expect_str_max(reader, maxLen);
+  mpack_error_t err = mpack_reader_error(reader);
+  if (err != mpack_ok) {
+    return err;
+  }
+  const char* bytes = mpack_read_bytes_inplace(reader, count);
+  if (bytes) {
+    out->assign(bytes, count);
+  } else {
+    return mpack_reader_error(reader);
+  }
+  mpack_done_str(reader);
+  return mpack_ok;
+}
+
+mpack_error_t mpack::mpack_read_str(mpack_reader_t* reader, mpack_tag_t* tag,
+                                    std::string* out, uint32_t maxLen) {
+  uint32_t count = mpack_tag_str_length(tag);
+  mpack_error_t err = mpack_reader_error(reader);
+  if (err != mpack_ok) {
+    return err;
+  }
+  if (count > maxLen) {
+    mpack_reader_flag_error(reader, mpack_error_too_big);
+    return mpack_error_too_big;
+  }
+  const char* bytes = mpack_read_bytes_inplace(reader, count);
+  if (bytes) {
+    out->assign(bytes, count);
+  } else {
+    return mpack_reader_error(reader);
+  }
+  mpack_done_str(reader);
+  return mpack_ok;
+}