Note:
* Exp2 doesn't have a special case for very small arguments
* Exp2 hasn't been subject to a proper error analysis
Also:
* add tests for Exp2 with integer argument
* always test Go versions of Exp and Exp2
R=rsc
CC=Charlie Dorian, PeterGo, golang-dev
https://golang.org/cl/3481041
Eoghan Sherry