[RFC]: add `blas/ext/base/dcombinations`

[kgryte]

This API is similar to the Cartesian product and the following APIs found elsewhere

• https://docs.pytorch.org/docs/stable/generated/torch.combinations.html
• https://www.mathworks.com/help/matlab/ref/nchoosek.html

API signature:

function dcombinations( N, k, replacement, x, strideX, out, strideOut1, strideOut2 )

where out is a 2-dimensional strided array which is assumed to hold at least N choose k elements if replacement is false and N+k-1 choose k elements if replacement is true.

This (and similar (s|z|c|g) APIs will set the stage for an eventual blas/ext/combinations API.

Because out is two-dimensional, the implementation should be cache-aware (i.e., set elements in out in a cache-optimal fashion).