Adds impl Tokonizable for Bytes

[chabashilah]

Hi,

I wanted to use Bytes type as a return value from smart contract calling but I got a following error.

error[E0277]: the trait bound `(web3::types::Bytes, web3::types::Bytes): web3::contract::tokens::Detokenize` is not satisfied
  --> src/bin/call_func_check_data.rs:48:40
   |
48 |     let retval:(Bytes, Bytes) = result.wait().unwrap();
   |                                        ^^^^ the trait `web3::contract::tokens::Detokenize` is not implemented for `(web3::types::Bytes, web3::types::Bytes)`
   |
   = help: the following implementations were found:
             <(A, B) as web3::contract::tokens::Detokenize>
   = note: required because of the requirements on the impl of `web3::futures::Future` for `web3::contract::QueryResult<(web3::types::Bytes, web3::types::Bytes), web3::transports::shared::Response<fn(hyper::body::chunk::Chunk) -> std::result::Result<serde_json::value::Value, web3::Error>, hyper::body::chunk::Chunk>>`

I send a PR to deal with the issule. Could you check #313 ?

[tomusdrw]

Closed via #313