go/ast: fix printing of maps
go/ast: fix printing of maps

Print a newline after each map entry;
similar to the style used for slices
and structs.

R=r, r2
CC=golang-dev
http://codereview.appspot.com/4274042
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go