`Owl.Dense.Ndarray.S.copy_` is 3-4x slower than `Owl_base_dense_ndarray_generic.copy_`

[nathanfarlow]

Hello,
Owl.Dense.Ndarray.S.copy_ is ~3-4x slower than Owl_base_dense_ndarray_generic.copy_ because it doesn't call Genarray.blit. Is this intentional/necessary? Can Owl_dense_ndarray_generic.copy_ be updated to use Owl_base_dense_ndarray_generic.copy_? Below are some benchmarks:

open Core
open Core_bench

let sizes = [ 10; 100; 1000; 10000; 100000; 1000000; 10000000 ]

let make_bench size =
  let name = Printf.sprintf "Copy 1D array of size %d" size in
  let src = Owl.Dense.Ndarray.S.empty [| size |] in
  let dst = Owl.Dense.Ndarray.S.empty [| size |] in
  Bench.Test.create ~name (fun () -> Owl.Dense.Ndarray.S.copy_ src ~out:dst)
;;

let () =
  let tests = List.map sizes ~f:make_bench in
  Command_unix.run (Bench.make_command tests)
;;

Owl_dense_ndarray_generic.copy_
┌────────────────────────────────┬────────────────┬─────────┬────────────┐
│ Name                           │       Time/Run │ mWd/Run │ Percentage │
├────────────────────────────────┼────────────────┼─────────┼────────────┤
│ Copy 1D array of size 10       │       103.61ns │   4.00w │            │
│ Copy 1D array of size 100      │       114.55ns │   4.00w │            │
│ Copy 1D array of size 1000     │       291.18ns │   4.00w │      0.01% │
│ Copy 1D array of size 10000    │     2_006.74ns │   4.00w │      0.09% │
│ Copy 1D array of size 100000   │    18_705.19ns │   4.00w │      0.80% │
│ Copy 1D array of size 1000000  │   185_585.75ns │   4.00w │      7.97% │
│ Copy 1D array of size 10000000 │ 2_327_796.01ns │   4.00w │    100.00% │
└────────────────────────────────┴────────────────┴─────────┴────────────┘

Owl_base_dense_ndarray_generic.copy_
┌────────────────────────────────┬──────────────┬─────────┬────────────┐
│ Name                           │     Time/Run │ mWd/Run │ Percentage │
├────────────────────────────────┼──────────────┼─────────┼────────────┤
│ Copy 1D array of size 10       │      94.01ns │  36.00w │      0.02% │
│ Copy 1D array of size 100      │      97.32ns │  36.00w │      0.02% │
│ Copy 1D array of size 1000     │     115.62ns │  36.00w │      0.02% │
│ Copy 1D array of size 10000    │     639.84ns │  36.00w │      0.11% │
│ Copy 1D array of size 100000   │   5_578.15ns │  36.00w │      0.95% │
│ Copy 1D array of size 1000000  │  61_613.86ns │  36.00w │     10.49% │
│ Copy 1D array of size 10000000 │ 587_095.10ns │  36.00w │    100.00% │
└────────────────────────────────┴──────────────┴─────────┴────────────┘