Document public `rustic_core` API

[simonsan]

A good rolemodel here is IMHO: https://github.com/BurntSushi/aho-corasick/blob/master/src/lib.rs

aho-corasick/src/lib.rs has a complete abstract about:

• [ ] what the library does
• [ ] what is the basis for it (e.g. also the restic repository format might be worthwhile to name and link here)
• [ ] available configuration options
• [ ] overview of primary types of the library
  • [ ] ready to paste examples for different use cases linking to all essential types in the docs with an
  • [ ] abstract explaining what it (example) actually does
• [ ] lower level implementation details/APIs
• [ ] crate features