API improvements

[sz-piotr]

The current API is not ideal and has several problems including:

• Function names from abi can override doppelganger properties and methods e.g. deploy,
• Overloaded methods are not supported on the Doppelganger object,
• It is possible to cause exceptions because the user called Doppelganger methods in an incorrect order (calling .returns(...) before deploy(),
• Setup requires calling multiple methods.

To address some of those issues the following API can be used:

const abi = [
  'function transfer(address,uint256) public returns(bool)',
  'function foo() public returns(uint256,string)',
  'function bar(uint256 x) public pure returns (uint256)',
  'function bar(bool x) public pure returns (bool)',
];

async function example() {
  const doppelganger = new Doppelganger(abi);
  const { address } = await doppelganger.deploy();
  await doppelganger.methods.transfer.returns(true);
  await doppelganger.methods.transfer.returns(42, 'hello');
  // support for overloaded names. Requires identifier
  await doppelganger.methods['bar#0423a132'].returns(1337);
}

The changes are as follows:

• deploy returns the contract instance, making it easy to get the address and the contract safely,
• all of the abi methods names now reside in the doppelganger.methods object,
• a special syntax name#hex is used for overloaded methods

However I believe some further improvements can be made if we allow some bigger changes. Let's look at the following example:

const abi = [
  'function transfer(address,uint256) public returns(bool)',
  'function foo() public returns(uint256,string)',
  'function bar(uint256 x) public pure returns (uint256)',
  'function bar(bool x) public pure returns (bool)',
];

async function example() {
  // create initially. Deploys and sets up methods
  const doppelganger = await Doppelganger.from(abi, {
    transfer: true,
    foo: [42, 'hello'],
    'bar#0423a132': 1337, // support for overloaded names. Requires identifier
  });

  // access members
  const usedAbi = doppelganger.abi;
  const contract = doppelganger.contract;
  const address = doppelganger.address;

  // change something later
  await doppelganger.mockReturn('transfer', false);
  await doppelganger.mockReturn('foo', [21, ':37']);
}

This approach addresses all of the pain points listed at the start of this issue, by deploying the contract in the constructor (that is no longer strictly class based).