Closes #1627

This keeps it out of the main include path when benchmarking is
enabled, somewhat reducing the compilation-time penalty.

Also moved some other functions into the .cpp file, especially
helpers that could be given internal linkage, and concretized some
iterator-templated code that only ever used
`std::vector<double>::iterator`.

Closes #1581

This allows us to move <stdexcept> out of the common path, and replace
it with just <exception>. The difference between these two headers is
~13k lines after preprocessing on libstdc++ (16k vs 3k) and ~17k lines
for MS's STL(33k vs 16k).

Note that this is only beneficial if no other stdlib header we use
includes <stdexcept>. AFAIK this is true for the newest MS's STL,
but I have no idea of the applicability for libstdc++ and libc++.

* Units from <ratio> are no longer redeclared in our own namespace
* The default clock is `steady_clock`, not `high_resolution_clock`,
because, as HH says "high_resolution_clock is useless. If you want
measure the passing of time, use steady_clock. If you want user
friendly time, use system_clock".
* Benchmarking support is opt-in, not opt-out, to avoid the large
(~10%) compile time penalty.
* Benchmarking-related options in CLI are always present, to decrease
the amount of code that is only compiled conditionally and making
the whole shebang more maintainble.