Correct encoding of the Solidity storage layout in the Expr translation

[zoep]

The translation of Act to Expr is not faithful to the Solidity storage layout for values shorted than a word. The current translation naively puts every storage value in 256bit word slots. As a result, equivalence for the following fails:

constructor of A
interface constructor()

iff

  CALLVALUE == 0

creates

  uint128 x := 11
  uint128 y := 42

contract A {
    uint128 x;
    uint128 y;
    
    constructor() {
        x = 11;
        y = 42;
    }
}

Correct encoding of the layout will also fix #175.

[zoep]

Note that this will make the order of declaring storage variables in Act relevant. An alternative would be for the user to provide the storage layout in the specification or read the layout from the compiler generated json, so that we don't make compiler-specific assumptions.