Custom callbacks#4

This will help implement complex types such as vector of dynamic strings or multidimensional vectors with dynamic dimensions. 1. Custom grow functions (or maybe macros instead, more effecient?). 2. In-place constructors and destructors for elements. 3. Custom memory management (something like allocators in C++). To save memory in such cases when callbacks aren't needed, pointers to them should be derived into another struct and pointer to that struct should be stored in header. If it's `NULL`, then we don't use them. ~~Forget about assignment callbacks: we don't need them here at all. In addition, for assignment callbacks we also need typecast callbacks, that's out of scope of this library.~~ In far and happy future: 1. Custom hash functions (for maps based on hashes). 2. ~~Custom comparison functions (for maps based on trees).~~