Make performance and clarity updates for C++17

[trws]

@kab163 did the great work to get everything behaving and requiring C++17 for all compilers in #171. There are a number of workarounds in camp to support C++14 compilers, and in some cases even 11, and we currently don't document the required version. This is partially just documentation, but should also include using things like actual fold expressions to implement the appropriate folds to save compile times.