Allow waiting for a specified number of confirmations for contract transactions.
This change only requires an optional TransactionResponse return type to be added to the contract function. This allows the transaction hash to be passed to `.wait`.
For example, previously the `mint` method looked like this without a return value:
```
method mint(token: TestToken, holder: Address, amount: UInt256) {.base, contract.}
```
it still works without a return value, but if we want to wait for a 3 confirmations, we can now define it like this:
```
method mint(token: TestToken, holder: Address, amount: UInt256): ?TransactionResponse {.base, contract.}
```
and use like this:
```
let receipt = await token.connect(signer0)
.mint(accounts[1], 100.u256)
.wait(3) # wait for 3 confirmations
```
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