Validate constructors in ancestor contracts

[spalladino]

Fixes #1332

[spalladino]

Funny thing. The GSNRecipient from contracts-ethereum-package extends from Context, which has an internal constructor.

And apparently Solidity generates an ABI entry for the constructor (even if it's internal), which is being picked up by the new validation, and causes the integration test to fail.

$ jq .abi build/contracts/Context.json 
[{
    "inputs": [],
    "payable": false,
    "stateMutability": "nonpayable",
    "type": "constructor"
}]

From the test log:

  1) Unpack a GSN Kit on the development
       Create a Counter proxy on the development:
     Error: Error running npx oz create Counter --init initialize --args 2 --network development (err 1)
undefined
- Compiling contracts with Truffle, using settings from truffle.js file
- Compiling contracts with Truffle, using settings from truffle.js file
- Compiling contracts with Truffle, using settings from truffle.js file
- - Contract Counter or an ancestor has a constructor. Change it to an initializer function. See https://docs.openzeppelin.com/sdk/2.6/writing-contracts#initializers.
- One or more contracts have validation errors. Please review the items listed above and fix them, or run this command again with the --force option.

      at execInWorkdir (/home/circleci/openzeppelin/tests/util/share.js:62:13)
      at run (/home/circleci/openzeppelin/tests/util/share.js:78:24)
      at Context.<anonymous> (src/integration.test.js:68:7)

We'll have to think how to get around this. Perhaps adding an exception for the Context contract? Or checking if the ctor body is empty?

[frangio]

There can be false positives if there is a "modifier" that is a super call to a parent contract with an empty constructor. But that's an edge case that I don't think will happen.

[frangio]

Looks good! There's linter errors though.