clang-13 in C++20 mode fails:
```
similar/main/newmenu.cpp:202:18: error: ISO C++20 considers use of overloaded operator '==' (with operand types 'exact_type<const dcx::grs_font>' and 'exact_
type<const dcx::grs_font>') to be ambiguous despite there being a unique best viable function [-Werror,-Wambiguous-reversed-operator]
return &cv_font == &game_font ? "\202" : "\207"; // 135
~~~~~~~~ ^ ~~~~~~~~~~
common/include/pack.h:31:17: note: ambiguity is between a regular call to this operator and a call with the argument order reversed
constexpr bool operator==(const T *rhs) const { return p == rhs; }
^
```
Add an operator== that exactly matches the input types so that clang
does not attempt a conversion.