Added optional feature mem-dbg

[LucaCappelletti94]

As per the title, I have added mem_dbg as an optional feature to make it easy to compare the memory requirements of this library with others.

Cheers!

Luca

[sundy-li]

CI is not passed, I revert this pr. You can send another pr to improve it.

cargo test --all-features    

rror[E0425]: cannot find function `size_of` in this scope
   --> /Users/sundy/.cargo/registry/src/github.com-1ecc6299db9ec823/mem_dbg-0.2.4/src/impl_mem_size.rs:672:17
    |
672 |                 size_of::<usize>()
    |                 ^^^^^^^ not found in this scope
    |

[LucaCappelletti94]

Which Rust version are you using?

[LucaCappelletti94]

Apparently they have already fixed this issue in mem-dbg, only the guy that owns the derive variant of the crate is in vacation and have yet to publish it.

[LucaCappelletti94]

I will make a new PR when it is published.