Meadowcap + Willow data model (sans store)

Hello! This very first release includes three new crates:

• meadowcap - A capability system for controlling access to Willow data.
• willow-data-model - The datatypes of Willow, an eventually consistent data store with improved distributed deletion.
• willow-encoding - Utilities for implementing Willow’s various encodings.

All of the above are fully documented.

What to expect inside: meadowcap is feature-complete, and willow_data_model has nearly everything: datatypes, groupings, encoding implementations. All it's missing is some definition of a store to actually store and retrieve all that Willow data from. Which is what we'll be working on next.

Many thanks to:

• @mycognosist
• @AljoschaMeyer
• The Iroh team (in particular, @Frando)
• @DerickEddington
• And NLnet for the funding which makes this work possible!