Semantics of functions returning multiple values

[bitbart]

Extend the semantics and TinySol so to handle functions with multiple return values:

https://docs.soliditylang.org/en/latest/control-structures.html#destructuring-assignments-and-returning-multiple-values

For example, the contract:

contract C {
    int x;
    bool b;

    function f() public view returns (int,bool) { return(x,b); }
    function g() public { int y; bool z; (y,z) = this.f(); x +=y; b = !z; }
}

has a function f that returns a pair of values. The returned pair can be deconstructed through an assignment when calling the function, as done in g.