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

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 <roland@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
Run-TryBot: Filippo Valsorda <filippo@golang.org>