net: make IP implement encoding.MarshalerText, encoding.UnmarshalerText

See golang.org/s/go12encoding for design.

R=golang-dev, bradfitz, mikioh.mikioh
CC=golang-dev
https://codereview.appspot.com/12705043
time: make Time implement encoding interfaces

See golang.org/s/go12encoding for design.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/12706043
cmd/5g: fix temp-merging on ARM

mkvar was taking care of the "LeftAddr" case,
effectively hiding it from the temp-merging optimization.

Move it into prog.c.

R=ken2
CC=golang-dev
https://codereview.appspot.com/12884045