Support my_array[::-1] to reverse an array

[suranap]

Example of failure:

>>> import cunumeric as np
>>> a = np.arange(10000)
>>> a[::-1]
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "mambaforge/envs/legate/lib/python3.1/site-packages/legate/core/runtime.py", line 2149, in wrapper
    result = func(*args, **kwargs)
             ^^^^^^^^^^^^^^^^^^^^^
  File "projects/legate/cunumeric/cunumeric/coverage.py", line 120, in wrapper
    return func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^
  File "projects/legate/cunumeric/cunumeric/array.py", line 148, in wrapper
    return func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^
  File "projects/legate/cunumeric/cunumeric/array.py", line 1053, in __getitem__
    return ndarray(shape=None, thunk=self._thunk.get_item(key))
                                     ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "projects/legate/cunumeric/cunumeric/deferred.py", line 995, in get_item
    result = self._get_view(key)
             ^^^^^^^^^^^^^^^^^^^
  File "projects/legate/cunumeric/cunumeric/deferred.py", line 900, in _get_view
    k, store = self._slice_store(k, store, dim + shift)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "projects/legate/cunumeric/cunumeric/deferred.py", line 550, in _slice_store
    store = store.slice(dim, k)
            ^^^^^^^^^^^^^^^^^^^
  File "mambaforge/envs/legate/lib/python3.1/site-packages/legate/core/store.py", line 1353, in slice
    raise NotImplementedError(f"Unsupported slicing: {sl}")
NotImplementedError: Unsupported slicing: slice(None, None, -1)
>>> b = np.flip(a)

Maybe this code could check if step == -1 and run np.flip instead.
https://github.com/nv-legate/legate.core/blob/08da13fc544f3db26bf1ef7ce9bdb85e72a9d9fb/legate/core/store.py#L1353