Update to new version of compact encodable and use it Clean up encoding implementations

[cowlicks]

This updates the crate to use the updated version of the compact encoding crate. The main parts:

• Rewrites of the CompactEncoding implementations. There are a lot of these but they are fairly simple.
• Removes the HypercoreState struct. This is no longer needed due to changes to upstream compact encoding. This was used downstream in the hypercore-protocol crate (fixed in this PR).
• Rewriting of src/oplog/mod.rs::ValidateLeaderOutcome this struct heavily used the HypercoreState struct so rewriting it was more involved.
• This also removes the re-exports of compact-encoding from encoding.rs I believe this was added when the compact-encoding crate was created because hypercore-protocol depndended on the encoding code here before it was moved to it's own crate

[ttiurani]

Looks pretty straightforward to me!