fast exp/log/tanh

[Boscop]

In case the goal of this crate is to include more fast functions (which I hope), here is some inspiration:
https://www.kvraudio.com/forum/viewtopic.php?f=33&t=464290
https://github.com/herumi/fmath
https://github.com/herumi/xbyak
https://varietyofsound.wordpress.com/2011/02/14/efficient-tanh-computation-using-lamberts-continued-fraction/#more-1929

I'm not sure if these are the best/fastest (for writing audio DSP plugins). It's always a tradeoff between accuracy and speed.
It would be useful to have them in Rust too, when writing audio DSP plugins in Rust.