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

Also add unit tests.

wpimath/src/main/native/cpp/geometry/struct/Pose2dStruct.cpp

@@ -0,0 +1,32 @@
+// 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/Pose2dStruct.h"
+
+namespace {
+constexpr size_t kTranslationOff = 0;
+constexpr size_t kRotationOff =
+    kTranslationOff + wpi::Struct<frc::Translation2d>::kSize;
+}  // namespace
+
+using StructType = wpi::Struct<frc::Pose2d>;
+
+frc::Pose2d StructType::Unpack(std::span<const uint8_t, kSize> data) {
+  return frc::Pose2d{
+      wpi::UnpackStruct<frc::Translation2d, kTranslationOff>(data),
+      wpi::UnpackStruct<frc::Rotation2d, kRotationOff>(data),
+  };
+}
+
+void StructType::Pack(std::span<uint8_t, kSize> data,
+                      const frc::Pose2d& value) {
+  wpi::PackStruct<kTranslationOff>(data, value.Translation());
+  wpi::PackStruct<kRotationOff>(data, value.Rotation());
+}
+
+void StructType::ForEachNested(
+    std::invocable<std::string_view, std::string_view> auto fn) {
+  wpi::ForEachStructSchema<frc::Translation2d>(fn);
+  wpi::ForEachStructSchema<frc::Rotation2d>(fn);
+}