Generic ndarray types.

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I need to make a generic NDArray type in C99 to help with refactoring libtcod's FOV and pathfinding algorithms. TCOD_Map does not work how I want it to since it combines FOV and pathfinding in an unnecessary way.

I've messed around with several C++ implementations so far: 1 2 3 4 and what I think I've been missing from these is that I should've focused more on views into arrays more. Having a good generic constexpr view type which various storages can convert into will reduce duplicate code and complexity while not having much of a performance penalty.

For C99 I'll have to mess with a handful of hard-coded numeric types. I also want at least one view-like object that takes callbacks instead of strided memory which will remove any limitations on what can be used as a view. I'll wrap all of this up in a union type and maybe make all functions interacting with this inline.