unique_impl_ptr: fix bug in operator<

[meator]

EDIT: Also fixed both operator<s for std::nullptr_t.

A non-const std::less instantiation cannot be used for const data.

Sample program showcasing the problem:

#include "spimpl.h"

int main()
{
    auto first = spimpl::make_unique_impl<int>();
    auto second = spimpl::make_unique_impl<int>();
    first < second;
}

gcc error output

In file included from test.cpp:1:
spimpl.h: In instantiation of 'bool spimpl::operator<(const unique_impl_ptr<T, D>&, const unique_impl_ptr<T2, D2>&) [with T1 = int; D1 = void (*)(int*); T2 = int; D2 = void (*)(int*)]':
test.cpp:7:13:   required from here
spimpl.h:274:31: error: no match for call to '(std::less<int*>) (spimpl::unique_impl_ptr<int, void (*)(int*)>::const_pointer, spimpl::unique_impl_ptr<int, void (*)(int*)>::const_pointer)'
  274 |         return std::less<CT>()(l.get(), r.get());
      |                ~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/13.2/bits/unique_ptr.h:37,
                 from /usr/include/c++/13.2/memory:78,
                 from spimpl.h:31:
/usr/include/c++/13.2/bits/stl_function.h:451:7: note: candidate: 'constexpr bool std::less<_Tp*>::operator()(_Tp*, _Tp*) const [with _Tp = int]' (near match)
  451 |       operator()(_Tp* __x, _Tp* __y) const _GLIBCXX_NOTHROW
      |       ^~~~~~~~
/usr/include/c++/13.2/bits/stl_function.h:451:7: note:   conversion of argument 2 would be ill-formed:
spimpl.h:274:46: error: invalid conversion from 'spimpl::unique_impl_ptr<int, void (*)(int*)>::const_pointer' {aka 'const int*'} to 'int*' [-fpermissive]
  274 |         return std::less<CT>()(l.get(), r.get());
      |                                         ~~~~~^~
      |                                              |
      |                                              spimpl::unique_impl_ptr<int, void (*)(int*)>::const_pointer {aka const int*}

clang error output

test.cpp:7:11: warning: relational comparison result unused [-Wunused-comparison]
    7 |     first < second;
      |     ~~~~~~^~~~~~~~
In file included from test.cpp:1:
./spimpl.h:274:16: error: no matching function for call to object of type 'std::less<CT>' (aka 'less<int *>')
  274 |         return std::less<CT>()(l.get(), r.get());
      |                ^~~~~~~~~~~~~~~
test.cpp:7:11: note: in instantiation of function template specialization 'spimpl::operator<<int, void (*)(int *), int, void (*)(int *)>' requested here
    7 |     first < second;
      |           ^
/usr/bin/../lib64/gcc/x86_64-unknown-linux-gnu/13.2/../../../../include/c++/13.2/bits/stl_function.h:451:7: note: candidate function not viable: 1st argument ('const_pointer' (aka 'const int *')) would lose const qualifier
  451 |       operator()(_Tp* __x, _Tp* __y) const _GLIBCXX_NOTHROW
      |       ^          ~~~~~~~~
1 warning and 1 error generated.