Added optional support for mem_dbg crate

[LucaCappelletti94]

As per the title, I have added optional support for the mem_dbg crate, a rust library to estimate accurately and quickly the size of objects. The reason I needed to put it here on Hasher objects and the likes is that several libraries that I am benchmarking use this library and have the structs from this library within their structs.

By adding this derive, it becomes trivial to measure accurately their memory requirements.