Precision testing

[gnzlbg]

Part of the testing harness should be to test the worst case precision of the APIs. Probably, the easiest way to do this would be to use the mpfr library as a dev-dependency and use it to check that all APIs have a worst-case precision of 0.5 ULPs. For the APIs taking multiple f32s or f64s we can't feasibly test them for all inputs, but these should at least be exhaustively tested for the inputs that generally trigger the worst-case precision.

[tgross35]

Done, only took 6 years :) #311