message: use derive_more to automate From impls for Event
diff: use crate::coordinator::{Coordinator, RoundParameters};
  +use derive_more::From;
  use sodiumoxide::crypto::box_;
  use std::{
  collections::HashMap,
  @@ -63,6 +64,7 @@ impl Service {
  }
  /// An event handled by the coordinator
  +#[derive(From)]
  pub enum Event {
  /// A message from a participant.
  Message(Message),
  @@ -81,27 +83,6 @@ pub enum Event {
  SeedDict(SeedDictRequest),
  }
  -impl From<RoundParametersRequest> for Event {
  - fn from(req: RoundParametersRequest) -> Self {
  - Self::RoundParameters(req)
  - }
  -}
  -impl From<SumDictRequest> for Event {
  - fn from(req: SumDictRequest) -> Self {
  - Self::SumDict(req)
  - }
  -}
  -impl From<SeedDictRequest> for Event {
  - fn from(req: SeedDictRequest) -> Self {
  - Self::SeedDict(req)
  - }
  -}
  -impl From<Message> for Event {
  - fn from(msg: Message) -> Self {
  - Self::Message(msg)
  - }
  -}
  -
  /// Event for an incoming message from a participant
  pub struct Message {
  /// Encrypted message
  