Add a Constexpr zero-initializing constructor for divider

[sharkautarch]

Usecase:

For function-local static divider objects, and especially for (c++20 or later) static constinit divider objects. making the default constructor constexpr using a Constexpr zero-initializing constructor might sometimes result in better codegen

Otherwise, the compiler might make the static variable be dynamically initialized the first time the initializer statement is reached at runtime. https://en.cppreference.com/w/cpp/language/storage_duration#Static_local_variables

Here's a godbolt link that demonstrates better codegen w/ constexpr default initialization: https://godbolt.org/z/s6xoK5xEe where latch(int) is normal static default initialization, and latch_constinit(int) is constinit static default initialization (constinit only works w/ constexpr constructors)

EDIT: I realized there were two problems with my original PR:

• constexpr constructor needs all members to be initialized
• the default constructor is purposefully empty, so that default-initializing an array of divider objects is fast

So instead of trying to make the default constructor constexpr, I've just added a constexpr constructor that takes a nullptr and zero-initializes the members.

which should allow for something like this: static constinit libdivide::divider<int64_t> static_div{nullptr};

I guess something like std::nullopt would be less awkward, but that would mean requiring another standard library header

[ridiculousfish]

Thank you!