Events from Receipts

[gakonst]

Ideally for the following contract's events:

contract C {
    event X(...);
    event Y(...);
}

The resulting codegen should be

#[derive(EthEvent)]
struct X {
   ...
}

#[derive(EthEvent)]
struct Y {
  ...
}

#[derive(EthEvent)]
enum XY {
       X(X),
       Y(Y),
}

// parse_logs will try to decode each individual log inside the receipt as either X or Y 
let ev: Vec<XY> = ethers_contract::parse_logs(tx_receipt)?;

It should be possible to use the enum as a union type to parse event logs of various types, allowing us to e.g. loop over a transaction's logs. Also, it'd be nice if we had this in a function like all_events (see here for the macro def).