Make `stride_rank` public and document

[Tokazama]

We have the tooling to in place to solve #332. I think JuliaSIMD uses stride_rank too. So I figured it would make sense to just make this a public function and document it. @chriselrod, I'm pretty sure you were the original author on this so if there's something else to add to the documentation that would be welcomed.

[codecov[bot]]

Codecov Report

Merging #334 (491696e) into master (8d2830f) will not change coverage. The diff coverage is 50.00%.

@@           Coverage Diff           @@
##           master     JuliaArrays/ArrayInterface.jl#334   +/-   ##
=======================================
  Coverage   90.11%   90.11%           
=======================================
  Files           9        9           
  Lines        1345     1345           
=======================================
  Hits         1212     1212           
  Misses        133      133           

Impacted Files	Coverage Δ
src/stridelayout.jl	89.60% <50.00%> (ø)

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[ChrisRackauckas]

Anything static should go to https://github.com/JuliaArrays/StaticArrayInterface.jl