Commit e26fcd7 causes VS 2017 compiler to crash about units

[nholthaus]

Yeah that commit needs to be rolled back. I was trying to use some more of Microsoft's "hidden friend" technique to reduce the linux compile time, and I was able to shave about 40% off the build time, but then visual studio completely blew up. BTW - internal compiler error means a problem with MSVC, not the library per se, not that it really matters.

The idea was: if I can move the nls operators into the class itself as hidden friends, I can reduce the amount of operator overload lookup, and simultaneously simplify the template enable_if checks, rendering them less expensive. It worked pretty well with gcc.

You may want to stick with v2.3.0 RC 2 for a while. The dev branch is going to be pretty experimental between now and RC 3, if there is one, while I attempt to reduce build time and see how badly I can break the compilers. RC 2 + some documentation updates may well end up being the final v2.3.0 release anyway.

[nholthaus]

This commit has been reverted.