Minor cleanup

[bansan85]

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.