[wpimath] Add protobuf/struct for trivial types (#5935)

This implements de/serialization for the types that aren't templated (SwerveDriveKinematics) in C++ or where there is no trivial way to go round-trip (Splines) for the messages.

wpimath/src/main/native/cpp/controller/struct/ArmFeedforwardStruct.cpp

@@ -0,0 +1,33 @@
+// 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/controller/struct/ArmFeedforwardStruct.h"
+
+namespace {
+constexpr size_t kKsOff = 0;
+constexpr size_t kKgOff = kKsOff + 8;
+constexpr size_t kKvOff = kKgOff + 8;
+constexpr size_t kKaOff = kKvOff + 8;
+}  // namespace
+
+using StructType = wpi::Struct<frc::ArmFeedforward>;
+
+frc::ArmFeedforward StructType::Unpack(std::span<const uint8_t, kSize> data) {
+  return frc::ArmFeedforward{
+      units::volt_t{wpi::UnpackStruct<double, kKsOff>(data)},
+      units::volt_t{wpi::UnpackStruct<double, kKgOff>(data)},
+      units::unit_t<frc::ArmFeedforward::kv_unit>{
+          wpi::UnpackStruct<double, kKvOff>(data)},
+      units::unit_t<frc::ArmFeedforward::ka_unit>{
+          wpi::UnpackStruct<double, kKaOff>(data)},
+  };
+}
+
+void StructType::Pack(std::span<uint8_t, kSize> data,
+                      const frc::ArmFeedforward& value) {
+  wpi::PackStruct<kKsOff>(data, value.kS());
+  wpi::PackStruct<kKgOff>(data, value.kG());
+  wpi::PackStruct<kKvOff>(data, value.kV());
+  wpi::PackStruct<kKaOff>(data, value.kA());
+}

wpimath/src/main/native/cpp/controller/struct/DifferentialDriveWheelVoltagesStruct.cpp

@@ -0,0 +1,26 @@
+// 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/controller/struct/DifferentialDriveWheelVoltagesStruct.h"
+
+namespace {
+constexpr size_t kLeftOff = 0;
+constexpr size_t kRightOff = kLeftOff + 8;
+}  // namespace
+
+using StructType = wpi::Struct<frc::DifferentialDriveWheelVoltages>;
+
+frc::DifferentialDriveWheelVoltages StructType::Unpack(
+    std::span<const uint8_t, kSize> data) {
+  return frc::DifferentialDriveWheelVoltages{
+      units::volt_t{wpi::UnpackStruct<double, kLeftOff>(data)},
+      units::volt_t{wpi::UnpackStruct<double, kRightOff>(data)},
+  };
+}
+
+void StructType::Pack(std::span<uint8_t, kSize> data,
+                      const frc::DifferentialDriveWheelVoltages& value) {
+  wpi::PackStruct<kLeftOff>(data, value.left.value());
+  wpi::PackStruct<kRightOff>(data, value.right.value());
+}

wpimath/src/main/native/cpp/controller/struct/ElevatorFeedforwardStruct.cpp

@@ -0,0 +1,34 @@
+// 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/controller/struct/ElevatorFeedforwardStruct.h"
+
+namespace {
+constexpr size_t kKsOff = 0;
+constexpr size_t kKgOff = kKsOff + 8;
+constexpr size_t kKvOff = kKgOff + 8;
+constexpr size_t kKaOff = kKvOff + 8;
+}  // namespace
+
+using StructType = wpi::Struct<frc::ElevatorFeedforward>;
+
+frc::ElevatorFeedforward StructType::Unpack(
+    std::span<const uint8_t, kSize> data) {
+  return frc::ElevatorFeedforward{
+      units::volt_t{wpi::UnpackStruct<double, kKsOff>(data)},
+      units::volt_t{wpi::UnpackStruct<double, kKgOff>(data)},
+      units::unit_t<frc::ElevatorFeedforward::kv_unit>{
+          wpi::UnpackStruct<double, kKvOff>(data)},
+      units::unit_t<frc::ElevatorFeedforward::ka_unit>{
+          wpi::UnpackStruct<double, kKaOff>(data)},
+  };
+}
+
+void StructType::Pack(std::span<uint8_t, kSize> data,
+                      const frc::ElevatorFeedforward& value) {
+  wpi::PackStruct<kKsOff>(data, value.kS());
+  wpi::PackStruct<kKgOff>(data, value.kG());
+  wpi::PackStruct<kKvOff>(data, value.kV());
+  wpi::PackStruct<kKaOff>(data, value.kA());
+}