Following up my D practices, I've created a 'groupBy' template[1] for
Stockman (my practice project).

I'd like to ask you more experienced folks to please take a look at it.
 As this is my first template, I'd like to know if I am doing anything
idiomatically/logically wrong.

PS: I've also implemented two simple function templates (all[1] and contains[2]) because the versions from dlib didn't appeal to my sense of usage beauty :-)

[1] https://github.com/bahmanm/stockman-d/blob/master/source/rangeutils.d#L38 [2] https://github.com/bahmanm/stockman-d/blob/master/source/rangeutils.d#L105 [3] https://github.com/bahmanm/stockman-d/blob/master/source/rangeutils.d#L129

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Bahman