Facilitate use of designated initializers for structs such as various Options

[pdillinger]

We have a number of P.O.D. Options structs that have unfortunate constructors with (positional) parameters that are easily used confusingly, as in ReadOptions{true, false}. Importantly, in C++20, the existence of this constructors interferes with use of the much healthier designated initializer syntax.

We should review our public APIs and deprecate these constructors, to promote better code hygiene. Along the same lines, we should avoid the pattern of describing default values in a comment, which could become out of sync with constructor code, and use default member initializers for defaults.