GitLab

Implement an iterator over CAR blocks that accepts both v1 and v2. The
reader only requires io.Reader interface to operate and exposes the
version and roots of the underlaying CAR file as fields.

Remove `Next` from CARv2 reader now that the iteration functionality is
provided using `BlockReader`.

Update the benchmarks to reflect the changes and add example usage for
`NewBlockReader`.

Add tests that assert `BlockReader` behaviour in erroneous and
successful cases for both underlying CARv1 and CARv2 payload.