changeTokenBalance should check that balanceOf has the right signature

[fvictorio]

See #4735

While the docs specify that these helpers only work with ERC20 token, the error you get if you use them with an ERC1155 token are very confusing. This is easy to improve on our side: we should add a check that contract.balanceOf.fragment.inputs has only one element of type uint256, or throw an error otherwise.