refactor(ipld): use Set/GetCell API from rstm2d

[Wondertan]

The latest rsmt2d version allows us to set cells/share directly into the imported square. Hence, we can avoid using flattened slices and the complexity it introduces when calculating position or index to set a share. Also, we now avoid reimporting the square on each repair retry and overall optimizing the whole process.

Closes #824 Blocked on https://github.com/celestiaorg/rsmt2d/issues/127 and https://github.com/celestiaorg/rsmt2d/pull/128

[Wondertan]

The race tests will fail here until https://github.com/celestiaorg/rsmt2d/pull/128 is merged and released

[Wondertan]

Rebased on main