Feature: constexpr default constructor

[burnpanck]

Although in an older issue, it was concluded that general constexpr may be difficult to achieve, I believe there would be considerable benefit in just a constexpr default constructor for the container types and the hooks. Since C++20, we have the constinit specifier for global variables, which means the global can be initialised without running code, which is desirable especially in bare-metal/kernel scenarios, where there is little one can rely on during early boot. However, constinit requires a constexpr constructor, for obvious reasons. Global containers in bare-metal scenarios usually start-off empty, so a default constructor that is constexpr would already go a long way.