FFI implementation for mobile

[sgreenbury]

Develop a foreign function interface (FFI) for mobile app.

[sgreenbury]

Next steps for debugging vc_verify_credential():

• Ensure credential can be received with latest trustchain-http crate
• Implement error handling as in FFI implementation for desktop #63
• Check verification is ok when signature only is true
• Confirm core_config().root_event_time is read
• Modify verification of issuer DID to use trustchain-http bundle endpoint
• Complete integration of initial light client IONVerifier (after Implement type state pattern for IONVerifier #101)