[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/struct/ChassisVelocitiesStructTest.cpp

@@ -0,0 +1,28 @@
+// 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"
+
+using namespace wpi::math;
+
+namespace {
+
+using StructType = wpi::util::Struct<wpi::math::ChassisVelocities>;
+const ChassisVelocities kExpectedData{
+    ChassisVelocities{2.29_mps, 2.2_mps, 0.3504_rad_per_s}};
+}  // namespace
+
+TEST(ChassisVelocitiesStructTest, Roundtrip) {
+  uint8_t buffer[StructType::GetSize()];
+  std::memset(buffer, 0, StructType::GetSize());
+  StructType::Pack(buffer, kExpectedData);
+
+  ChassisVelocities unpacked_data = StructType::Unpack(buffer);
+
+  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());
+}