Turbopack: bincode: Add custom bincode reader/writer implementations (#86632)
- We need a custom writer because we want to write to a `SmallVec`, and I don't want to go through `std::io::Write` to do that, as there's potential error checking overhead that's not needed (might compile away, but I don't want to take any chances).
- We need a custom reader because bincode's reader doesn't allow underlying access to the buffer mid-read.