The code in this directory is from https://github.com/herumi/fmath

It includes code for exp (and log for float32) that is much faster than the regular
std library exp function.

The only file here that we actually use is fmath.hpp, but we include the entire directory
to make sure we give full credit and provide full documentation.  Plus it's only 300K for
everything, so it doesn't add much to the size of the repo.