Minor cleanup

After reading some part of the library, I noticed some minor change with ABI break.

1. unverified_safe_because is template<size_t N> but N is never used.

Another declaration could be:

inline auto unverified_safe_because(const char *reason)

2. const -> static constexpr

The library is explicitly c++17. There is lots of const int CompileErrorCode = 42; that could be replaced by static constexpr int CompileErrorCode = 42;

3. in #define helper_create_converted_field, isFrozen is unused

So new declaration could be:

#define helper_create_converted_field(fieldType, fieldName)

4. Finally a question: the library is explicitly not thread-safe.

So why there is a std::mutex callback_lock, RLBOX_SHARED_LOCK, RLBOX_ACQUIRE_SHARED_GUARD and RLBOX_ACQUIRE_UNIQUE_GUARD? Is this a try to make a thread-safe library?

Thanks,

PS: If you're fine with these changes, I can implement them.

Start by locating unverified_safe_because, the CompileErrorCode declarations, helper_create_converted_field, and the callback locking macros. Determine whether removing unused parameters or changing declarations preserves the ABI, then resolve the question of why the mutex and shared or unique guards exist. Done means the cleanup scope and thread-safety behavior are explicitly decided.