gob: add support for maps.

Because maps are mostly a hidden type, they must be
implemented using reflection values and will not be as
efficient as arrays and slices.

R=rsc
CC=golang-dev
https://golang.org/cl/1127041

src/pkg/gob/codec_test.go

@@ -572,6 +572,7 @@ func TestEndToEnd(t *testing.T) {
 	s2 := "string2"
 	type T1 struct {
 		a, b, c int
+		m       map[string]*float
 		n       *[3]float
 		strs    *[2]string
 		int64s  *[]int64
@@ -579,10 +580,13 @@ func TestEndToEnd(t *testing.T) {
 		y       []byte
 		t       *T2
 	}
+	pi := 3.14159
+	e := 2.71828
 	t1 := &T1{
 		a:      17,
 		b:      18,
 		c:      -5,
+		m:      map[string]*float{"pi": &pi, "e": &e},
 		n:      &[3]float{1.5, 2.5, 3.5},
 		strs:   &[2]string{s1, s2},
 		int64s: &[]int64{77, 89, 123412342134},
@@ -921,6 +925,7 @@ type IT0 struct {
 	ignore_g string
 	ignore_h []byte
 	ignore_i *RT1
+	ignore_m map[string]int
 	c        float
 }
 
@@ -937,6 +942,7 @@ func TestIgnoredFields(t *testing.T) {
 	it0.ignore_g = "pay no attention"
 	it0.ignore_h = []byte("to the curtain")
 	it0.ignore_i = &RT1{3.1, "hi", 7, "hello"}
+	it0.ignore_m = map[string]int{"one": 1, "two": 2}
 
 	b := new(bytes.Buffer)
 	NewEncoder(b).Encode(it0)