Special Case: Contract event emits with no values

[bh2smith]

Currently when generating contract from abi for an event with no output values (like this one)

The resulting contract looks like this

            ) -> Result<Self, self::ethcontract::tokens::Error> {
                let () = self::ethcontract::tokens::Tokenize::from_token(token)?;
                Ok(FactoryDisabled())
            }

Which causes the following error/warning:

   Compiling contracts v0.1.0 (/Users/bensniff/Projects/cowprotocol/services/crates/contracts)
error: this let-binding has unit value
   --> /Users/bensniff/Projects/cowprotocol/services/target/debug/build/contracts-85b524e76e657330/out/BalancerV2StablePoolFactoryV2.rs:432:17
    |
432 |                 let () = self::ethcontract::tokens::Tokenize::from_token(token)?;
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: omit the `let` binding: `self::ethcontract::tokens::Tokenize::from_token(token)?;`
    |
    = note: `-D clippy::let-unit-value` implied by `-D warnings`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#let_unit_value

We could probably work around this with a clippy-ignore statement of some kind, but may also want to address this case head on.

I suspect there is some part of this file that will need to be updated

https://github.com/gnosis/ethcontract-rs/blob/dcbfbe594f65eb2cc9e22a5468b7212ad1aa2a39/ethcontract-generate/src/generate/events.rs#L1

For more context: see cowprotocol/services#371