Benchmark if passing "const std::string &" or just "std::string" is better

[olly]

Most modern compilers seem to implement std::string using PIMPL, so the class is just a single pointer and pass-by-value may be more efficient.

MSVC is the notable exception I'm currently aware of. Macro magic is one possible solution. Or perhaps benchmarking will reveal moving helps non-MSVC without penalising MSVC much.

Either way, most of the library passes by const ref, but some of the internals pass by value, and we should standardise!

[olly]

We're certainly not going to change the internals of 1.0.x in such a sweeping way at this point, but it would be worth benchmarking this before 1.1.0 if we can.