Support static mapping assignments

[stiefn]

If a value is assigned to a mapping using a constant as key, the compiler optimizes the keccak() opcode out. This means, we cannot find the mapping slot to a given slot the value is assigned to.

Minimal testing case:

contract Test1 {
    mapping(address account => uint256) bla;

    constructor() {
        bla[address(1)] = 500;
    }
}