(Closes #2381) alternative approach to global reduction

[arporter]

Refactors the existing support for global sums and begins adding support for global max/min.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.95%. Comparing base (16639e7) to head (653c9bd).
⚠️ Report is 126 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3222   +/-   ##
=======================================
  Coverage   99.95%   99.95%           
=======================================
  Files         380      380           
  Lines       53916    53916           
=======================================
  Hits        53894    53894           
  Misses         22       22           

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[arporter]

The testing was becoming increasingly artificial as I didn't have any "legitimate" way to introduce kernels with gh_min or gh_max access. Therefore I've started looking at adding the actual builtin(s) that will use these new accesses. #2674 requests support for field_min_max so I've begun adding that. However, I've then tripped over the fact that we currently enforce the requirement for a kernel/builtin to have only 1 or 0 reductions and min-max needs 2.

[arporter]

Somehow or other I lost the changes on this branch so I'm closing it.