P2944R3: Constrained equality - `std::optional`

[Zingam]

• https://eel.is/c++draft/optional.relops
  https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/p2944r3.html#pnum_15
• https://eel.is/c++draft/optional.comp.with.t
  https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/p2944r3.html#pnum_21