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 neg;

void main()
{
    int i = 32;
    long l = 55;
    float f = 23;
    double d = 4;
    assert(-i == -32);
    assert(-l == -55);
    assert(-f == -23);
    assert(-d == -4);
}