PermutedDimsArray, and layout for dense 1st index

[mcabbott]

Closes #10

Not entirely sure whether SecondMajor will be needed in the end. (Nor what named things should have.)

[codecov]

Codecov Report

Merging #12 into master will increase coverage by 0.20%.
The diff coverage is 64.28%.

@@            Coverage Diff             @@
##           master      #12      +/-   ##
==========================================
+ Coverage   61.14%   61.34%   +0.20%     
==========================================
  Files           8        8              
  Lines         700      727      +27     
==========================================
+ Hits          428      446      +18     
- Misses        272      281       +9     

Impacted Files	Coverage Δ
src/ArrayLayouts.jl	42.25% <ø> (ø)
src/memorylayout.jl	68.50% <64.28%> (-0.33%)	⬇️

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[dlfivefifty]

I don't think this needs to be in ArrayLayouts.jl: I tend to avoid union types due to compile time type inference issues (though something this simple won't be impacted to that) and its only used in one place.

If there's a strong reason to keep it, it needs a more descriptive name.

[mcabbott]

OK, I removed this for now.

The argument for it is that you would almost never want to dispatch on UnitStride{1} and exclude ColumnMajor, it just happens that there's no abstract type which fills this role.

[dlfivefifty]

Hmm, I'm tempted to remove IncreasingStrides and define

const ColumnMajor = UnitStride{1}

though perhaps some more thought is needed...

[mcabbott]

Do you mean ColumnMajor = Union{DenseColumnMajor, UnitStride{1}} as a replacement for AbstractColumnMajor?

One could also re-organise the type tree so that these two share a supertype. Does anything care about increasing strides, as opposed to just DenseColumnMajor or UnitStride{1}?

However I also need UnitStride{2}, and clearly the tree can't depend on D.

[dlfivefifty]

Looks good! If you address the minor comments and get the code coverage tests passing I'll merge.