math: improve accuracy of Exp2

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
6 files changed