net: reimplement toLower to not depend on strings
net: reimplement toLower to not depend on strings

R=golang-dev, r, bradfitz
CC=golang-dev, jas
https://codereview.appspot.com/43610043

Committer: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang