strconv: even faster int conversion
strconv: even faster int conversion

benchmark                           old ns/op    new ns/op    delta
strconv_test.BenchmarkFormatInt         10038         8217  -18.14%
strconv_test.BenchmarkAppendInt          6822         4969  -27.16%
strconv_test.BenchmarkFormatUint         2811         1814  -35.47%
strconv_test.BenchmarkAppendUint         2349         1360  -42.10%

R=rsc
CC=golang-dev
http://codereview.appspot.com/5488083

wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang-old