experiment: erc20 minting from a Go contract

[mattkanwisher]

We had some sample code around using the vm from Go. In future revision of the DpoS contracts we will need them to mint erc20. So lets add a dummy method on dpos3 that can mint tokens on an erc20, using the open Zeppelin mint table method. It should be secured in such a way that only the chain can do the mint.

1. Make simple erc20 with openzeppelin mintable https://github.com/OpenZeppelin/openzeppelin-solidity/blob/master/contracts/token/ERC20/ERC20Mintable.sol
2. Add a method to dpos3 that mints a token
3. Figure out a secure strategy of making the chain send an "from"/"sender" address that only the chain can use

[enlight]

The Gateway Go contracts can be used for reference on how to mint ERC20 from a Go contract.
What we need is to add a new MintVouchers method to the DPOS contract that can be called only by delegators. MintVouchers should take a BigUInt amount and mint that amount of tokens to the DPOS contract, then transfer them to the caller.

[suhasagg]

PR for the same -
#1334

Go-loom PR for the same -
loomnetwork/go-loom#426