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

Also add unit tests.

wpimath/src/main/native/cpp/geometry/struct/Rotation2dStruct.cpp

@@ -0,0 +1,22 @@
+// 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 "frc/geometry/struct/Rotation2dStruct.h"
+
+namespace {
+constexpr size_t kValueOff = 0;
+}  // namespace
+
+using StructType = wpi::Struct<frc::Rotation2d>;
+
+frc::Rotation2d StructType::Unpack(std::span<const uint8_t, kSize> data) {
+  return frc::Rotation2d{
+      units::radian_t{wpi::UnpackStruct<double, kValueOff>(data)},
+  };
+}
+
+void StructType::Pack(std::span<uint8_t, kSize> data,
+                      const frc::Rotation2d& value) {
+  wpi::PackStruct<kValueOff>(data, value.Radians().value());
+}