Use new `eachrow` and `eachcol`

[arnavs]

Now that https://github.com/JuliaLang/julia/pull/29749 has been merged, we can use for row in eachrow(M) and for col in eachcol(M) etc., instead of for i in axes(M, 2), M[:, i] or whatever.

Will need to get this pre-compatted first though, unless we use Julia 1.1

[arnavs]

Also eachslice(M, dims) for arbitrary sizes.