Only allow unchecked()/mutable_unchecked() on an lvalue

[jagerman]

This should mitigate accidental invocation on a temporary array.

Fixes #961.

[dean0x7d]

Seems fine to me -- preferable to keeping the underlying object alive via refcount. References/views in C++ never do that anyway. Even more so in our case, since the type itself (detail::unchecked_reference) is an implementation detail, the reference isn't expected to be kept around and the type isn't meant to be named.

As far as I can see, the only use case that might be prevented by the lvalue qualifier is:

py::array_t<double> get_array();
void foo(detail::unchecked_reference ref);

foo(get_array().unchecked<1>()); // valid temporary lifetime

But this isn't very likely, especially because of the detail:: part.