Crate containing all the components for implementing the Settlement
Architecture for the Interledger Protocol. The crate is structured such that
an API is created by giving it an object which implements the
SettlementEngine trait. All settlement engines must be implemented under the
engines subdirectory, with a directory name describing their
functionality, e.g. ethereum_ledger, ethereum_unidirectional_channel,
Trait consumed by the Settlement Engine HTTP API. Every settlement engine
MUST implement this trait, so that it can be then be exposed over the API.