[wpimath] Replace Speeds with Velocities (#8479)

I left "free speed" alone since that's the technical term for it. In
general, velocity is a vector quantity, and speed is a magnitude (i.e.,
a strictly positive value).

This PR also replaces the speed verbiage in MotorController with duty
cycle.

Fixes #8423.

wpimath/src/test/native/cpp/kinematics/proto/ChassisVelocitiesProtoTest.cpp

@@ -0,0 +1,29 @@
+// 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 <gtest/gtest.h>
+
+#include "wpi/math/kinematics/ChassisVelocities.hpp"
+#include "wpi/util/SmallVector.hpp"
+
+using namespace wpi::math;
+
+namespace {
+
+const ChassisVelocities kExpectedData =
+    ChassisVelocities{2.29_mps, 2.2_mps, 0.3504_rad_per_s};
+}  // namespace
+
+TEST(ChassisVelocitiesProtoTest, Roundtrip) {
+  wpi::util::ProtobufMessage<decltype(kExpectedData)> message;
+  wpi::util::SmallVector<uint8_t, 64> buf;
+
+  ASSERT_TRUE(message.Pack(buf, kExpectedData));
+  auto unpacked_data = message.Unpack(buf);
+  ASSERT_TRUE(unpacked_data.has_value());
+
+  EXPECT_EQ(kExpectedData.vx.value(), unpacked_data->vx.value());
+  EXPECT_EQ(kExpectedData.vy.value(), unpacked_data->vy.value());
+  EXPECT_EQ(kExpectedData.omega.value(), unpacked_data->omega.value());
+}