go spec: specify constant conversions
go spec: specify constant conversions

This is not a language change.

Added paragraphs specifying which conversions
yield results that are constants.

R=r, rsc, iant, ken
CC=golang-dev
http://codereview.appspot.com/4515176

tv42/old-go,apparentlymart/golang,dsriram/golang,gnanderson/go,tv42/old-go,dsriram/golang,pombredanne/go-deleteme,tv42/old-go,gnanderson/go,ssrl/go,ssrl/go,ssrl/go,gnanderson/go,apparentlymart/golang,ssrl/go,gnanderson/go,pombredanne/go-deleteme,ssrl/go,dsriram/golang,tv42/old-go,ssrl/go,gnanderson/go,gnanderson/go,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,dsriram/golang,dsriram/golang,apparentlymart/golang,dsriram/golang,pombredanne/go-deleteme,dsriram/golang,gnanderson/go,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,ssrl/go,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme