Returning a PendingTransaction after sending a tx

[gakonst]

Improves the ergonomics of sending & waiting for a transaction to be mined. You can now do

// Before
let tx_hash = provider.send_transaction(...).await?
dbg!(tx_hash);
let receipt = provider.pending_transaction(tx_hash).await?;

// After:
let receipt = provider.send_transaction(...).await?.await?;

// or:
let pending_tx = provider.send_transaction(...).await?;
dbg!(*pending_tx);
let receipt = pending_tx.await?;

// you can still use the syntax for N confs like so:
let receipt = provider.send_transaction(...).await?.confirmations(6).await?;

THIS IS A BREAKING CHANGE

• If you use previously used the tx_hash in the return values of a send_transaction call, you should deref it since PendingTransaction derefs to TxHash
• Smart contract calls need to be split in the "create call" and "send call":

// Before:
let pending_tx = contract
            .method::<_, H256>("setValue", "hi".to_owned())
            .unwrap()
            .send()
            .await
            .unwrap();

// After:
let call = contract.method::<_, H256>("setValue", "hi".to_owned()).unwrap();
let pending_tx = call.send().await.unwrap();

[gakonst]

Merging, CI failure is related to #105