This CL also replaces similar loops in other stdlib
package tests with calls to AllocsPerRun.
Fixes#4461.
R=minux.ma, rsc
CC=golang-dev
https://golang.org/cl/7002055
Fixes#3495.
I adapted fmt.TestCountMallocs to fix the
existing tests. As the resulting tests did not
appear to belong to either itoa or ftoa I moved
them into their own file.
R=bradfitz, fullung
CC=golang-dev
https://golang.org/cl/5985072