compiler: make struct types more unique

There are a lot more fields that are important when comparing structs
with each other. Take them into account when building the unique ID per
struct type.

Example code that differs between the compilers:
https://play.golang.org/p/nDX4tSHOf_T

testdata/interface.go

@@ -11,6 +11,10 @@ func main() {
 	printItf(*thing)
 	printItf(thing)
 	printItf(Stringer(thing))
+	printItf(struct{ n int }{})
+	printItf(struct {
+		n int `foo:"bar"`
+	}{})
 	printItf(Number(3))
 	array := Array([4]uint32{1, 7, 11, 13})
 	printItf(array)
@@ -47,6 +51,14 @@ func printItf(val interface{}) {
 		println("is Thing:", val.String())
 	case *Thing:
 		println("is *Thing:", val.String())
+	case struct{ i int }:
+		println("is struct{i int}")
+	case struct{ n int }:
+		println("is struct{n int}")
+	case struct {
+		n int `foo:"bar"`
+	}:
+		println("is struct{n int `foo:\"bar\"`}")
 	case Foo:
 		println("is Foo:", val)
 	default: