Consider using a tensor lib for native memory (instead of flat slices)

[jonysy]

• ndarray

[jonysy]

alexandermorozov:

ndarray solves first part by accepting constant values, slices, Vecs and iterators as initializers. But it doesn't allow to allocate uninitialized chunk of memory to be overwritten later; looks like there is no way to statically check if usage is correct, so checks have to be done at runtime and that'll have a small runtime cost.