message: Add documentation to FfiResult
diff: use snafu::Snafu;
  use std::fmt::Debug;
  +/// A result type for calling into the backend via FFI. Such a call causes
  +/// non-negligible computational cost and must always have gas information
  +/// attached. In order to prevent new calls from forgetting such gas information
  +/// to be passed, the inner success and failure types contain gas information.
  pub type FfiResult<T> = core::result::Result<FfiSuccess<T>, FfiError>;
  /// A return element and the gas cost of this FFI call
  