image/color/palette: use go generate to create palette.go

LGTM=r
R=golang-codereviews, r
CC=golang-codereviews
https://codereview.appspot.com/134940043

Committer: Rob Pike <r@golang.org>
strconv: use go generate to create isprint.go

LGTM=r
R=golang-codereviews, r
CC=golang-codereviews
https://codereview.appspot.com/132230043

Committer: Rob Pike <r@golang.org>