feat(api): Support sending assets from contract

[slipo]

Solves #147

This PR allows you to send assets from a contract. It supports Contract and Invocation transactions (which are used in neo-boa examples such as https://github.com/CityOfZion/neo-boa/blob/master/boa/tests/src/WithdrawTest.py#L100).

I saw very similar functionality for mintTokens. I'm unsure how mintTokens was being used in this case. If we need it to work as before, is there a way to test it? It will be easy to add back in the exact same way as before if needed.

For my implementation I set invocationScript to '00' * number of contract parameters which is what the mintTokens version was doing. I did not append the main script's invocationScript as the issue says as this does not seem to be needed. The original mintToken implementation was using the contract's script hash in the attribute and I needed to use the sender's. Again, it wasn't clear to me how mintToken was working. What we have here is much more like how neo-python works with --from-addr when sending.

Here is some testing code:
https://gist.github.com/slipo/c1542c087bff7e17a067e903f0b24540

Here's a resulting transaction:
https://gist.github.com/slipo/6b2815058652fce6b386267f1b880639

[snowypowers]

yes we need the mintToken functionality. Please update it such that the old functions are retained and make your own new ones. They serve very different functions although they look similar. The mintToken function allows the contract to reject the transaction if the token sale is oversubscribed / finished. We are adding features to allow control over SC assets.

Do make a distinction for a smart contract. A normal send asset transaction is called ContractTransaction because an address is considered a normal contract.

[snowypowers]

We want our mintToken functionality.

[snowypowers]

do use SC or smart contract instead of just contract. An normal address is a normal contract.

[slipo]

Thank you for the feedback. I've made all changes.

I see that mintTokens is a special case where the assets are coming from the sender's regular address but they want the verification triggered. I've put all original mintTokens code back.

I also have created a sendAssets example:
https://gist.github.com/slipo/b4ff4569ada4a93fce683c3da512e0f2

Please see my question in that code. It seems to me we need to (option 1, shown in example) pass along a scriptHash when doing this form of send assets. Otherwise we need to (option 2) remove this:

    if (config.address !== acct.address && !sendingAddressIsSmartContract(config)) {
      return Promise.reject(
        new Error('Private Key and Balance address does not match!')
      )
    }

Or a third option is to add a new config.sendingFromSmartContract = true. I would personally prefer options 1 or 3 because I think that this isn't the most common usage and that error message could be useful for people.

[snowypowers]

yes, I think adding a sendingFromSmartContract field would be the option here. We do not want to deviate from the original purpose of sending assets from a personal account. Another solution could be to create a new method called sendAssetsFromSC in order to differentiate. I will accept either methods for now.

[slipo]

Done. Looking cleaner to me.

[snowypowers]

Thanks for the work! There might be some changes as to where this should live but that should be minor.