Remove address overloads on contracts

[axic]

I think it would make sense removing those overloads and making their use explicit by typecasting to an address.

The reason is to make it explicit what functions are defined on a contract and not have confusing overloads, common examples are in the case of transfer and balance.

[axic]

It seems we will lose one feature: warning if the destination contract has a non-payable fallback function.