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

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