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
https://golang.org/cl/4274042

src/pkg/go/ast/print.go

@@ -158,6 +158,7 @@ func (p *printer) print(x reflect.Value) {
 			p.print(key)
 			p.printf(": ")
 			p.print(v.Elem(key))
+			p.printf("\n")
 		}
 		p.indent--
 		p.printf("}")