Added mem dbg and fixed how no-std is defined

[LucaCappelletti94]

I have added support as an optional feature to mem-dbg, a crate to accurately measure the size of structs. I need to measure the sizes of crates using siphash as part of their structs, hence the need for this pull request.

Plus, previously you had defined the no-std attribute as:

#![cfg_attr(not(test), no_std)]

But that way, if you requested the std feature, it would not actually disable no-std - It would only do that while testing. In order to do both I changed it to:

#![cfg_attr(all(not(test), not(feature = "std")), no_std)]

Cheers!