add method Value() Value to InterfaceValue.

use Value() in print to print underlying value
from interface.

before:
	package main
	import "fmt"
	func main() {
		x := []interface{} {1, "hello", 2.5};
		fmt.Println(x[0], x[1], x[2], x);
	}

	1 hello 2.5 [<non-nil interface> <non-nil interface> <non-nil interface>]

after:
	1 hello 2.5 [1 hello 2.5]

R=r
DELTA=44  (22 added, 16 deleted, 6 changed)
OCL=27139
CL=27141

src/lib/reflect/all_test.go

@@ -301,6 +301,16 @@ func TestInterfaceGet(t *testing.T) {
 	assert(v3.Type().String(), "float");
 }
 
+func TestInterfaceValue(t *testing.T) {
+	var inter struct { e interface{ } };
+	inter.e = 123.456;
+	v1 := reflect.NewValue(&inter);
+	v2 := v1.(reflect.PtrValue).Sub().(reflect.StructValue).Field(0);
+	assert(v2.Type().String(), "interface { }");
+	v3 := v2.(reflect.InterfaceValue).Value();
+	assert(v3.Type().String(), "float");
+}
+
 func TestCopyArray(t *testing.T) {
 	a := []int{ 1, 2, 3, 4, 10, 9, 8, 7 };
 	b := []int{ 11, 22, 33, 44, 1010, 99, 88, 77, 66, 55, 44 };