Fix slot overwrite on packed variables

[wei3erHase]

Bug description:

PR #117

When variables are declared nearby (sometimes either in other abstract contract, but close on declaration order), using the setVariable method on one, can erase the information about the other variable.

contract Bug {
  address public myAddress;
  bool public myBool;
}

await bug.setVariable('myAddress', randomAddress)
await bug.setVariable('myBool', true)

console.log(await bug.myAddress()) // => 0x0000...

Given that myAddress (that occupies ~uint160) and myBool are stored together, the second setVariable method aims to the entire data slot and overwrites it with 0x0000...1 (true), and when the myAddress slot is fetched, is filled with 0s.

An example contract is added, and a test replicating the descripted bug.