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

Also add unit tests.

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

@@ -0,0 +1,31 @@
+// 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/QuaternionStruct.h"
+
+namespace {
+constexpr size_t kWOff = 0;
+constexpr size_t kXOff = kWOff + 8;
+constexpr size_t kYOff = kXOff + 8;
+constexpr size_t kZOff = kYOff + 8;
+}  // namespace
+
+using StructType = wpi::Struct<frc::Quaternion>;
+
+frc::Quaternion StructType::Unpack(std::span<const uint8_t, kSize> data) {
+  return frc::Quaternion{
+      wpi::UnpackStruct<double, kWOff>(data),
+      wpi::UnpackStruct<double, kXOff>(data),
+      wpi::UnpackStruct<double, kYOff>(data),
+      wpi::UnpackStruct<double, kZOff>(data),
+  };
+}
+
+void StructType::Pack(std::span<uint8_t, kSize> data,
+                      const frc::Quaternion& value) {
+  wpi::PackStruct<kWOff>(data, value.W());
+  wpi::PackStruct<kXOff>(data, value.X());
+  wpi::PackStruct<kYOff>(data, value.Y());
+  wpi::PackStruct<kZOff>(data, value.Z());
+}