[arrays] rm TraversalOrder, Opt.

- Rm TraversalOrder, Opt according to meeting's decision.
- Fix slice for custom orders.
- Remove the options at compile time.
- Fix doc.
[arrays] group_indices for custom mem. order

- generalize the group_indices for the custom order.
[arrays] add c_ordered_transposed_view
Cleaning
[arrays] improve slice, special for ellipsis
[arrays] Back some TraversalOrder

- Tests show that it is necessary to have it at compile time.
- Put this flag back, simplified.
[arrays] assignement. Specialize one case for speed.

- also FORCEINLINE, according to speed test.
[arrays] add one speed test
[arrays] Modify iterators for better speed.

- along the lines decided for the foreach
[doc] Update array doc.