reflect: add SetZero

This was added in Go 1.20 and is required by encoding/json starting with
Go 1.21.

src/reflect/all_test.go

@@ -1559,8 +1559,6 @@ func TestIsZero(t *testing.T) {
 			t.Errorf("%d: IsZero(Zero(TypeOf((%s)(%+v)))) is false", i, x.Kind(), tt.x)
 		}
 
-		/* // TODO(tinygo): missing SetZero support
-
 		p := New(x.Type()).Elem()
 		p.Set(x)
 		p.SetZero()
@@ -1568,7 +1566,6 @@ func TestIsZero(t *testing.T) {
 			t.Errorf("%d: IsZero((%s)(%+v)) is true after SetZero", i, p.Kind(), tt.x)
 
 		}
-		*/
 
 	}