Upgrade rkyv to 0.8 and remove bytecheck

[ssfdust]

The rkyv 0.8 version is a major update that brings many improvements and new features. This patch adapts the API to version 0.8, removing the old API.

Additionally, the key-value type in the test cases has been changed from HashMap to BTreeMap, as the data bytes of HashMap are no longer fixed in version 0.8. I suspect this is due to the unordered sorting of HashMap.

After switching to BTreeMap, everything works fine.

[emmyoh]

@austinhartzheim Has this PR been noticed?

[austinhartzheim]

Thanks for the contribution upgrading to rkyv 0.8. At Cloudflare, we have several use-cases that require rkyv 0.7, so we cannot merge the PR as it stands today. Currently, this crate is tightly coupled to a single rkyv version, and I would like to break that coupling so it can be used across multiple rkyv versions.

Short-Term Plan:
We could add a read_raw() method, similar to the existing write_raw() method. That would allow you to read and write raw bytes and use rkyv at a version of your choosing, or any other deserialization logic to operate on those raw bytes.

Can you confirm that this would unblock your use-case?

Long-Term Plan:
Eventually, I believe we could make the interface generic over other zero-copy deserialization frameworks while preserving the typed API. Potentially, we can support multiple versions of rkyv with this solution, although further work is required to determine what the interface would look like.