Hopefully this provides the best of all worlds.Importantly, this re-introduces src/test.rs, as that's something thatcan't be trivially generated.Both approaches are documented so the path to the easiest workingsolution is clear for vendors, and they can pick either the moreexpensive solution, or the simpler (but less comprehensive) one.If this PR meets general approval, I'd like to add a .travis.ymlstep that automatically tests the --cfg minimal_tests path, so atleast, that will be a "stable" target, as testing that the deployedcrate is free from errors is presently too much work to expect theaverage maintainer or contributor to remember to do.Closes:unicode-rs#38

This is still just a work-in-progress, so may be rebased-out in future,just seeing if it Works yet before I spend time on something potentiallymore finely grained.