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.

[kab163]

FYI - @rhornung67 is going to talk to code team leads to see if they have any concerns about requiring c++17 and plans to bring it up at the PCC meeting next Wednesday.

[trws]

Thanks @kab163, if possible let me know how that goes.

[trws]

How did that conversation work out @rhornung67?

[kab163]

Is this issue still needed?

[rhornung67]

After the release, it may be worthwhile to get a group together to walk through the code and do some house cleaning related to older C++ standards like @trws mentioned at the top of this issue.