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

Also add unit tests.

wpimath/src/main/native/include/frc/geometry/struct/Rotation2dStruct.h

@@ -0,0 +1,21 @@
+// 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.
+
+#pragma once
+
+#include <wpi/SymbolExports.h>
+#include <wpi/struct/Struct.h>
+
+#include "frc/geometry/Rotation2d.h"
+
+template <>
+struct WPILIB_DLLEXPORT wpi::Struct<frc::Rotation2d> {
+  static constexpr std::string_view kTypeString = "struct:Rotation2d";
+  static constexpr size_t kSize = 8;
+  static constexpr std::string_view kSchema = "double value";
+
+  static frc::Rotation2d Unpack(std::span<const uint8_t, kSize> data);
+  static void Pack(std::span<uint8_t, kSize> data,
+                   const frc::Rotation2d& value);
+};