crypto/internal/edwards25519: reduce Point size by reordering fields

Updates #58483

Tested on Linux amd64:
  type Element struct {
    l0, l1, l2, l3, l4 uint64
  }

  type PointAfter struct {
    x, y, z, t Element
    _          incomparable
  }

  type PointBefore struct {
    _          incomparable
    x, y, z, t Element
  }

  type incomparable [0]func()

  func main() {
    fmt.Println(unsafe.Sizeof(PointAfter{})) // 168
    fmt.Println(unsafe.Sizeof(PointBefore{})) // 160
  }

Change-Id: I6c4fcb586bbf3febf62b6e54608496ff81685e43
Reviewed-on: https://go-review.googlesource.com/c/go/+/467616
Reviewed-by: Roland Shoemaker <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Filippo Valsorda <[email protected]>
Reviewed-by: Damien Neil <[email protected]>
Run-TryBot: Filippo Valsorda <[email protected]>

edwards25519.go

@@ -27,13 +27,13 @@ type projP2 struct {
 //
 // The zero value is NOT valid, and it may be used only as a receiver.
 type Point struct {
-	// The point is internally represented in extended coordinates (X, Y, Z, T)
-	// where x = X/Z, y = Y/Z, and xy = T/Z per https://eprint.iacr.org/2008/522.
-	x, y, z, t field.Element
-
 	// Make the type not comparable (i.e. used with == or as a map key), as
 	// equivalent points can be represented by different Go values.
 	_ incomparable
+
+	// The point is internally represented in extended coordinates (X, Y, Z, T)
+	// where x = X/Z, y = Y/Z, and xy = T/Z per https://eprint.iacr.org/2008/522.
+	x, y, z, t field.Element
 }
 
 type incomparable [0]func()