Use stricter C++ type conversions (#4357)

Now, implicit narrowing conversions are only used with wpi::Now(). This
also fixes clang-tidy warnings about C-style casts. For example:
```
== clang-tidy /__w/allwpilib/allwpilib/wpilibNewCommands/src/main/native/include/frc2/command/SwerveControllerCommand.inc ==
/__w/allwpilib/allwpilib/wpilibNewCommands/src/main/native/include/frc2/command/SwerveControllerCommand.inc:95:18: warning: C-style casts are discouraged; use static_cast/const_cast/reinterpret_cast [google-readability-casting]
  auto curTime = units::second_t(m_timer.Get());
                 ^
```
In that case at least, the cast was removed entirely since Get() already
returns a units::second_t.

wpilibc/src/test/native/cpp/simulation/StateSpaceSimTest.cpp

@@ -40,7 +40,7 @@ TEST(StateSpaceSimTest, FlywheelSim) {
   for (int i = 0; i < 100; i++) {
     // RobotPeriodic runs first
     auto voltageOut = controller.Calculate(encoder.GetRate(), 200.0);
-    motor.SetVoltage(units::volt_t(voltageOut) +
+    motor.SetVoltage(units::volt_t{voltageOut} +
                      feedforward.Calculate(200_rad_per_s));
 
     // Then, SimulationPeriodic runs