Generate higher order struct types

[raffopazzo]

The following program typechecks but the llvmgen stage terminates with the error `cannot emit program with higher order struct types. It should be possible to emit LLVM code by boxing fields with a dependent type.

func int_or_bool(bool_t which) -> typename
{
    if (which)
        return i32_t;
    else
        return bool_t;
}

struct t
{
    bool_t x;
    int_or_bool(x) y;
};

func f(t x) -> int_or_bool(x.x)
{
    return x.y;
}