Add clippy CI check to workflow

[andrewvious]

Closes #75

The clippy check isn't currently passing, working on getting the warnings taken care of in a separate PR.

[andrewvious]

There are only a few clippy warnings left revolving around a manual implementation of an assigned operation, function cannot return without recursing, and direct implementation of ToString.
These warning's can be handled in a separate PR need be @eureka-cpu @mitch-vrrb

[mitch-vrrb]

Those clippy errors aren't warnings, this test overflows the stack:

#[cfg(test)]
mod test {
    #[test]
    fn test_u256_into() {
        let eth_u256: ethereum_types::U256 = ethereum_types::U256::one();

        let token_u256: &mut super::U256 = eth_u256.into();
    }
}

[eureka-cpu]

Can you see if removing the recursive From implementations causes any failures? Left one other review comment, otherwise looks good.

[eureka-cpu]

This looks fine to me, the check doesn't necessarily need to pass for this to go in.