Move bigint serialization utils

[austinabell]

Summary of changes
Changes introduced in this pull request:

• moves bigint serialization to utils folder
  • I noticed the wrapper types serialization could be derived with self:: instead of that manual impl when doing this, so I made that change too for readability
• removes unused dev dependencies
• Adds Bytes serde wrappers (cleaner than going through ByteBuf and I found I mistyped .to_vec() instead of .into_vec() in a couple cases which is an unnecessary clone). This just makes usage easier and cleaner for the future.
  • Originally just wanted to add the serde(transparent) tag for cleaner expanded serde derivation, but then I noticed the other things
  • Also cleaned up other serde impls that I noticed

Just a couple small things that were bugging me and I wanted to clean up

Reference issue to close (if applicable)

Closes

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