Improve builder ergonomics, remove prefix

[divagant-martian]

A couple of improvements for ergonomics

• For an user of the builder, calling new("v4") provides little value taking into account v4 is the only supported identity scheme. So just implement Default for the builder instead,
• A more common and friendly pattern is to given access to the BuilderOfX via X::builder since that's an easily avoidable import, so now Enr provides: Enr::builder()
• In some cases it's desirable to get an empty Enr. This would be EnrBuilder::new().build(&key), there is a friendly shortcut for this Enr::empty
• Removes the Enr prefix from EnrBuilder this shouldn't be even needed to import thanks to the Enr::builder and Enr::empty

Notes

Tests don't use the most ergonomic versions of the builder in their current form (before this change). Since test setup is most of the time done by copying the setup from a related test, I updated them all to use the most appropriate, ergonomic version of what they had

Also, there is a clippy failure but I don't think having those warnings enabled makes sense:

• remove extra clippy warnings #54