Replace quadratic Reverse with quasilinear

[andralex]

Since alias sequences are stored internally as arrays, Reverse is quadratic because it builds an array by prepending an element. Each step creates a new array and copies the existing one into it.

The proposed implementation uses the quadratic algorithm for short arrays (where it is still efficient) and switches to a divide & conquer approach for larger arrays. Unless I'm wrong it does O(n) work for each level of the tree formed by halving, for a total of O(n long n) work. The short arrays case improves this by a constant. factor.

I think we ought to examine other compile-time algorithms with an eye for compile-time complexity. Also, per the #8039, this pattern should be detected and implemented so it has constant amortized complexity:

x = AliasSeq!(x, one_more_element);

[dlang-bot]

Thanks for your pull request, @andralex!

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Testing this PR locally

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dub run digger -- build "master + phobos#8301"

[andralex]

Wait, Reverse uses foreach_reverse to build the list by appending. I'll close this, sorry for the distraction.