boost::hana::drop_front doesn't respect refness

```
#include
#include
#include
#include
#include
#include
#include

#include
#include
#include

namespace hana = boost::hana;

int main()
{
int x = 1;
int y = 2;
int z = 3;

std::tuple refs {x, y, z};
auto ref_drop = boost::hana::drop_front(refs);

x = 10;
y = 20;
z = 30;

std::cout << x << " " << y << " " << z << std::endl;
std::cout << std::get<0>(refs) << " " << std::get<1>(refs) << " " << std::get<2>(refs) << std::endl;
std::cout << std::get<0>(ref_drop) << " " << std::get<1>(ref_drop) << std::endl;
}
```
The output of the above code is:
```
10 20 30
10 20 30
2 3
```
Expected output is:
```
10 20 30
10 20 30
20 30
```
I would expect if I pass `boost::hana::drop_front` a `std::tuple` to get back a `std::tuple` but instead I get a `std::tuple`. This is unexpected. Is this intended behavior?

Start by compiling and running the reproducer from the issue, then inspect boost::hana::drop_front and its std::tuple handling. Add a regression test covering std::tuple of references and verify that the dropped tuple preserves the remaining references and produces the expected output.

Written by the indexing model from the issue text.