feat: implement needed new utility methods

[ttiurani]

This implements the methods added recently to the Javascript project:

https://github.com/mafintosh/flat-tree

(All of these methods are needed in the v10 implementation of hypercore.)

The commits can be reviewed one by one, and contain the context.

Bonus:

• Bump edition to 2021 and version to next major
• Fix in Cargo.toml outdated information about github address and licence and bogus description
• Fix clippy warnings
• Move documentation to lib.rs.