Fix compilation with gcc16

[bmwiedemann]

Do not redefine the standard placement new/delete operators

tools.h defined the placement forms of operator new/delete itself instead of pulling them in from <new>. That only works as long as no standard library header defining them has been included before.

As of GCC 16, libstdc++ includes <new> from <bits/stl_iterator.h>, which cube.h drags in via <math.h> -> <cmath>, so the definitions in tools.h now collide with the ones from the standard library and the build fails:

In file included from shared/cube.h:57:
shared/tools.h:38:14: error: redefinition of 'void* operator new(size_t, void*)'
   38 | inline void *operator new(size_t, void *p) { return p; }
      |              ^~~~~~~~
/usr/include/c++/16/new:168:7: note: 'void* operator new(std::size_t, void*)' previously defined here

Include <new> and drop the hand-written definitions. The standard ones are inline and behave identically - and, unlike ours, are constexpr where the standard requires it. The custom operator new(size_t, bool) overloads are not affected.