`exp`, `log` and friends slow.

https://github.com/JuliaMath/DoubleFloats.jl/pull/136 gives `DoubleFloats` an `exp` (and `exp2`, `exp10`) implementation that is around 4x faster than what `MultiFloats` is using. If you wanted this version to work for all levels of precision, it would take some extra engineering, but IMO, it would be worth it.