Fixing mutual recursive types serialization

[tomusdrw]

Fixes below error:

error[E0275]: overflow evaluating the requirement `v1::types::trace::VMOperation: serde::Serialize`
    --> /home/tomusdrw/eth/parity/dapps/target/release/build/ethcore-rpc-2826bc3b63de4723/out/mod.rs:3647:18
     |
3647 |             impl _serde::ser::Serialize for VMOperation where
     |                  ^^^^^^^^^^^^^^^^^^^^^^
     |
     = note: required by `serde::Serialize`

error: aborting due to previous error

Issue: serde-rs/serde#441

[coveralls]

Coverage decreased (-0.01%) to 86.913% when pulling 3e3a08f on overflow-fix into f69b3f8 on master.