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/main/native/include/frc/interfaces/Gyro.h

@@ -77,7 +77,7 @@ class Gyro {
    *         based on integration of the returned rate from the gyro.
    */
   virtual Rotation2d GetRotation2d() const {
-    return Rotation2d{units::degree_t{-GetAngle()}};
+    return units::degree_t{-GetAngle()};
   }
 };