opam: pin bytesrw to the dbuenzli/bytesrw#13 of_slice fix

bytesrw 0.3.0 Reader.of_slice ignores a slice's first offset and reads from byte 0 (dbuenzli/bytesrw#13), so a Reader cannot address a sub-range of a larger buffer without a copy. The fix is on the bytesrw main branch but unreleased; pin to it in the root and in every package that depends on bytesrw directly, so the whole closure resolves one fixed version.