[wpimath, wpiutil] Add wpi::array for compile time size checking

[calcmogul]

The wpimath APIs use std::array, which doesn't do size checking. Passing
an array with the wrong size can result in uninitialized elements
instead of a compilation error.

[PeterJohnson]

Won't this let {} pass successfully even for a N-sized array?

[calcmogul]

Yea... in that case, we need to fix the places we use the default constructor of std::array/wpi::array.

[calcmogul]

I don't see a clean way of not using the default ctor in SwerveDriveKinematics.inc.
https://github.com/wpilibsuite/allwpilib/blob/master/wpimath/src/main/native/include/frc/kinematics/SwerveDriveKinematics.inc#L41-L53

[PeterJohnson]

How about a constructor tag to explicitly request an unfilled array? E.g.

struct empty_array_t {};
constexpr empty_array_t empty_array = empty_array_t{};

class array {
  public:
    explicit array(empty_array_t) {}

That forces the empty case to be explicitly constructed as array<X,Y> arr(empty_array)

[calcmogul]

I like this approach, but it is rather verbose.

  wpi::array<SwerveModuleState, NumModules> moduleStates{
      wpi::array<SwerveModuleState, NumModules>::empty_array};

It can't see the definition of empty_array without the extra qualification. We could move the type tag definition outside the class. Ah, the snippet changed. :)

[PeterJohnson]

If you put the empty_array struct outside the class (see my edited comment), you can avoid the redundant template info.