backport: Optimize chained hyperslab selection.

[1uc]

A common pattern for creating semi-unstructured selection is to use many
(small) RegularHyperSlab and chain them:

HyperSlab hyperslab;
for(auto slab : regular_hyper_slabs) {
  hyperslab |= slab;
}

This eventually triggers calling:

for(auto slab : regular_hyper_slabs) {
  auto [offset, stride, counts, blocks] = slab;
  H5Sselect_hyperslab(space_id, offset, stride, counts, block);
}

Measurements show that this has runtime that's quadratic in the number
of regular hyper slabs. This starts becoming prohibitive at 10k - 40k
slabs.

We noticed that H5Scombine_select does not suffer from the same
performance issue. This allows us to optimize (long) chain of Op::Or
using divide and conquer.

The current implementation only optimizes streaks of Op::Or.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.24%. Comparing base (84d9ce8) to head (a7b22df).

Additional details and impacted files

@@            Coverage Diff             @@
##             v2.x    #1032      +/-   ##
==========================================
+ Coverage   86.13%   86.24%   +0.10%     
==========================================
  Files          90       90              
  Lines        5858     5902      +44     
==========================================
+ Hits         5046     5090      +44     
  Misses        812      812              

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