Building off of ucan-wg/rs-ucan#105, temporarily targeting the GitHub branch.

Changes:

• Provide None hasher to witnessed_by
• Expiration can now be empty, handle
• Use generate_capability() where appropriate
• Facts are now always maps, key the link value under LINK_RECORD_FACT_NAME as "link" -- open to suggestions here, probably should be less generic than "link", though not completely noosphere-specific as these could be shared outside the nooverse

Rename:

• Capability => CapabilityView
• With => Resource
• Action => Ability
• Resource => ResourceUri
• SphereAction => SphereAbility