Add predecessor1 and predecessor0 (and successor) functions to RsVec

Similarly to how we exploit the rank/select structure in the `iter1()` implementation of `RsVec`, we can exploit it to find a predecessor/successor bit to any given index quickly (in the average case).

When implemented, this can be used to speed up the `delta(i)` implementation of Elias-Fano, replacing the second `select1` call with a `predecessor1` call.