make io.ReadFile use Stat.Size as a hint for preallocation
make io.ReadFile use Stat.Size as a hint for preallocation

R=rsc
CC=golang-dev
http://codereview.appspot.com/163069

nightlyone/golang,apparentlymart/golang,dsriram/golang,bdwalton/debian-golang,bdwalton/debian-golang,h8liu/golang,h8liu/golang,gnanderson/go,pombredanne/go-deleteme,tianon/debian-golang,tv42/old-go,bdwalton/debian-golang,nightlyone/golang,pombredanne/go-deleteme,ssrl/go,apparentlymart/golang,ssrl/go,tianon/debian-golang,dsriram/golang,apparentlymart/golang,tv42/old-go,tv42/old-go,nightlyone/golang,gnanderson/go,ssrl/go,tianon/debian-golang,dsriram/golang,apparentlymart/golang,apparentlymart/golang,ssrl/go,tianon/debian-golang,pombredanne/go-deleteme,bdwalton/debian-golang,h8liu/golang,tianon/debian-golang,ssrl/go,bdwalton/debian-golang,h8liu/golang,ssrl/go,dsriram/golang,dsriram/golang,gnanderson/go,bdwalton/debian-golang,tianon/debian-golang,pombredanne/go-deleteme,nightlyone/golang,nightlyone/golang,dsriram/golang,tv42/old-go,h8liu/golang,gnanderson/go,h8liu/golang,gnanderson/go,bdwalton/debian-golang,ssrl/go,apparentlymart/golang,nightlyone/golang,tv42/old-go,bdwalton/debian-golang,tianon/debian-golang,tv42/old-go,tianon/debian-golang,gnanderson/go,pombredanne/go-deleteme,tv42/old-go,h8liu/golang,pombredanne/go-deleteme,nightlyone/golang,dsriram/golang,gnanderson/go,apparentlymart/golang,pombredanne/go-deleteme