Added optional support for mem-dbg

[LucaCappelletti94]

Hi - I have added optional support for the mem-dbg crate, guarded behind the mem-dbg feature. I have also added optional support for enabling std, which is needed at this time for mem-dbg.

[eldruin]

Thank you for your work.
I do wonder if it is worth adding mem-dbg as dependency, though. It seems to me like a one-off check. I think that would be better done with a fork and [patch.crates-io].
Can you elaborate your use-case?

[LucaCappelletti94]

The gist of my use is that I am comparing several libraries that use different hash functions (stuff for set cardinality estimation) and testing how performance changes with different hashes.

Some of these libraries chose to have the Hasher or the HasherBuilder object not like a Phantom Type marker, but as an effective part of their struct and therefore it needs to be accounted for by mem_dbg. With the MemSize trait derived on the structs of this crate, the memory requirements computation becomes trivial. I have already merged pull requests in several of these aforementioned libraries adding an analogous feature flag, so integrating it also here would facilitate this considerably.

While I agree wholeheartedly that having it as a default dependency would be a rather poor life choice, I believe that having it as an optional dependency should be okay. Do you agree?