Fix VarDecls for tuples. Closes #99. I've implemented it this way since it doesn't require any changes in the frontend. However, I think having TypeTuple expressed as LLVM struct types would make much more sense and open the door to tuple lvalues.

module structs4;

struct S{
    int a;
    T t;
}

struct T{
    int b;
    U u;
}

struct U{
    int c;
}

void main()
{
    S s;
    s.a = 3;
    s.t = T.init;
    s.t.b = 4;
    s.t.u = U.init;
    s.t.u.c = 5;
    //{assert(s.t.u.c == 5);}
}