Switch to thiserror or snafu

libipld currently uses anyhow. Unfortunately:

- [According to the author](https://github.com/dtolnay/anyhow#comparison-to-thiserror), thiserror is a better fit for libraries. Anyhow makes it difficult to handle error cases.
- Anyhow doesn't have great nostd support as `?` will no longer convert errors into `anyhow::Error`.
- Anyhow requires allocation, adding an allocation to all error paths. While libipld itself requires allocation at the moment, it would be nice to eventually drop this requirement (neither the `Encode` nor `Decode` trait require allocations).