interledger-settlement API review for v1.0-rc

[gakonst]

interledger-settlement public items marked with x are documented. Non-exported items are not taken into account. Follows format from #561.

Part of #557.

• api (module)
• client::SettlementClient struct (need to also document its send_settlement method)
• SettlementMessageService (struct): passthrough unless message is targeted for our node's SE
• create_settlements_filter
• core (module)
• backends_common (module)
• engines_api (module)
• idempotency (module)
• types (module)
• NO_ENGINE_CONFIGURED_ERROR_TYPE (const)
• CONVERSION_ERROR_TYPE (const)
• SE_ILP_ADDRESS (const)
• Quantity (struct)
• ApiResponse(enum): Some APIs have default responses, e.g. Creating an account on the engine does not require us to get a response from the engine, so we use that to return a default value instead
• SettlementEngine (trait): trait is documented, could use some extension and docs on its functions
• SettlementEngineDetails (struct)
• SettlementAccount (trait)
• SettlementStore (trait)
• LeftoversStore (trait)
• ConvertDetails (struct)
• Convert (trait)
• scale_with_precision_loss (fn)
• IdempotentStore (trait)
• make_idempotent_call (fn): Should this be made pub(crate)?
• get_hash_of (fn) Should this be made pub(crate)? Note that this function is redefined in a few crates.
• CreateAccount
• create_settlement_engine_filter
• EngineRedisStore
• EngineRedisStoreBuilder