rework gminmax

[mattdowle]

• folds gmax into gmin becoming gminmax to avoid the duplicated logic so that future maintenance can be in one place
• reviewed the logic and changed it slightly; e.g. with default na.rm=FALSE, once an NA has occurred in a group, there is no need to fetch the value of the remaining values in that group
• moved R API outside loops
• removed the warnings (closes Remove "no non-missing" warning for character min and max? #4999) and it's now type consistent. I don't know why we ever thought it was a good idea to return Inf, it was just to be consistent with base
• news item with detailed example

There is some overlap with @MichaelChirico's #4445 here which fixes integer64 support in gmin/gmax. I'll merge that next which should be easier as it's now in one gminmax.

Merging so the NEWS item is published in dev and folk can test dev. Everything in dev is still open for discussion and reconsideration before release.

[codecov]

Codecov Report

Merging #5105 (dc1cfe6) into master (53278d3) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #5105      +/-   ##
==========================================
- Coverage   99.54%   99.54%   -0.01%     
==========================================
  Files          76       76              
  Lines       14691    14592      -99     
==========================================
- Hits        14624    14525      -99     
  Misses         67       67              

Impacted Files	Coverage Δ
src/gsumm.c	100.00% <100.00%> (ø)

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