Consider adding `RowRefMut<'_>` to reduce the amount of `unsafe`

[Centril]

To maintain sanity, we need to keep the growing amount of unsafe { ... } in table.rs under control. Consider adding a RowRefMut<'_> type to at least remove some get_row_ref_unchecked imposed by the borrow checker.

[Centril]

cc @gefjon -- Maybe something for 1.1 - Improve testing.