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.

wpilibcExamples/src/main/cpp/examples/DifferentialDrivePoseEstimator/include/Drivetrain.h

@@ -78,8 +78,8 @@ class Drivetrain {
   // Gains are for example purposes only - must be determined for your own
   // robot!
   frc::DifferentialDrivePoseEstimator m_poseEstimator{
-      frc::Rotation2d(),
-      frc::Pose2d(),
+      frc::Rotation2d{},
+      frc::Pose2d{},
       {0.01, 0.01, 0.01, 0.01, 0.01},
       {0.1, 0.1, 0.1},
       {0.1, 0.1, 0.1}};