Refactor CLI with traits and into separate crate

[sgreenbury]

Refactor the Trustchain CLI given #72:

• Move Trustchain CLI into separate crate trustchain-cli
• Construct traits (TrustchainDIDAPI and TrustchainVCAPI) for DID and VC functionality (Scope traits for specifying API #72)
• Implement traits for TrustchainAPI struct
• Merge trustchain-ffi crate from with gui and mobile modules from FFI implementation for mobile #62 and FFI implementation for desktop #63
• Add functions to be implemented for gui and mobile given Scope traits for specifying API #72
• Confirm both API traits' methods

[sgreenbury]

• Comment out todo methods of the API
• Consideer whether to remove trustchain-api dep on trustchain-ion given currently GUI and mobile implementations. If no trustchain-ion dep in trustchain-api crate, would need to:
  • Trait without default impl
  • Impl separately in CLI and FFI crates for local structs?

[sgreenbury]

Following consideration, trustchain-api remains downstream of trustchain-ion. Reconsider in further API refinements associated with #37.

[sgreenbury]

Closing with #80 merged.