Scope traits for specifying API

[sgreenbury]

Aim to implement a trait for each of the following API types currently in progress:

• CLI (include functions currently used but also consider ones missing)
• FFI (FFI implementation for mobile #62, FFI implementation for desktop #63): with consideration for available types
• HTTP (Server for issuing credentials #55, Server for verifiers of credentials/presentations #56): with consideration for functionality and response types.

[sgreenbury]

Proposed locations for API components:

• TrustchainDIDAPI trait within trustchain-ali crate (can't be core as uses did-ion)
• TrustchainVCAPI trait within trustchain-ali crate (can't be core as uses did-ion)
• TrustchainAPI struct implements above traits
• TrustchainMobileFFI convert trait to functions in a mobile module of the trustchain-ffi crate using TrustchainAPI struct
• TrustchainGUIFFI: convert trait to functions in a gui module of the trustchain-ffi crate using TrustchainAPI struct
• TrustchainHTTP, TrustchainIssuerHTTP and TrustchainVerifierHTTP: traits within the trustchain-http crate

[sgreenbury]

Superseded by #79