[FEA] Add a minmax reduction

[jrhemstad]

Is your feature request related to a problem? Please describe.

I would like to be able to compute the minimum and maximum element of a column in a single operation.

Describe the solution you'd like

A minmax reduction something like this:

std::pair<scalar, scalar> minmax(column_view const& c);

Describe alternatives you've considered

A dedicated minmax reduction is purely an optimization. The same functionality can be accomplished by just doing independent min and max reductions.

This idea was inspired by doing bounds checking for a gather map here.