Typechecker: functions returning multiple values

[bitbart]

Extend the typechecker of 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 w; int y; bool z; (w,y,z) = this.f(); x +=y; b = !z; }
}

should not pass typechecking, since the function f returns a pair of values, but the assignment refers to a triple.

The following contract should not pass typechecking as well:

contract C {
    int x;
    bool b;

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

since the type of the first element of the pair does not match with the return type of the function.