[wpimath] Move struct/proto classes to separate files (#5918)

Also add unit tests.

wpimath/src/test/native/cpp/geometry/struct/Transform3dStructTest.cpp

@@ -0,0 +1,28 @@
+// 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 <gtest/gtest.h>
+
+#include "frc/geometry/Transform3d.h"
+
+using namespace frc;
+
+namespace {
+
+using StructType = wpi::Struct<frc::Transform3d>;
+const Transform3d kExpectedData{
+    Transform3d{Translation3d{0.3504_m, 22.9_m, 3.504_m},
+                Rotation3d{Quaternion{0.3504, 35.04, 2.29, 0.3504}}}};
+}  // namespace
+
+TEST(Transform3dStructTest, Roundtrip) {
+  uint8_t buffer[StructType::kSize];
+  std::memset(buffer, 0, StructType::kSize);
+  StructType::Pack(buffer, kExpectedData);
+
+  Transform3d unpacked_data = StructType::Unpack(buffer);
+
+  EXPECT_EQ(kExpectedData.Translation(), unpacked_data.Translation());
+  EXPECT_EQ(kExpectedData.Rotation(), unpacked_data.Rotation());
+}