Support custom memory allocation functions

[me21]

At the moment, *_create functions use calloc. Please support custom memory (de)allocation functions like zlib-ng does.

[nmoinvaz]

Feel free to open a PR.

[tbeu]

Feel free to open a PR.

This is underspecified:

• Should it be at configuration-time, compile-time or run-time?
• If at run-time:
  • Should an additional user payload (opaque) (like in zlib or zlib-ng) be considered or should it be the same API as std::calloc and std::free?
  • Should existing API functions be kept for backwards compatibility or can they be altered?
• What about realloc?