First work on paginated tensors.

[chewxy]

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[ghost]

Thanks for opening this one up @chewxy!

I guess we want to integrate the paginated code following the existing file structure e.g., paginated_format.go, paginated_arith.go. Likely getting the code to follow closely to the Dense type.

Or do we want to try to integrate pagination as an option on a Dense tensor? It would probably just blow up the amount of code inside the Dense files.

EDIT: or we can just move everything into a new folder like you started.