[Linalg] Add a slice op

    This CL adds a linalg.slice op with the proper roundtripping test.
    A slice op allows taking subviews that may be rank-reducing (if some indexing is of index type) or not (if all indexings are of linalg.range type).

    A slice must be constructed directly from a base view (no chains of slices may exist in the IR). Helper functions that fold will be provided for construction if/when necessary.

    This also renames base_view to view.

--

PiperOrigin-RevId: 244406827