[parity-util-mem] consider using zeroize crate

[ordian]

https://github.com/iqlusioninc/crates/tree/develop/zeroize seems to be the best option available atm.

Also, https://github.com/iqlusioninc/crates/tree/develop/secrecy could replace our Secret type.

[dvdplm]

I looked at zeroize previously and I think it's a great crate and you are probably right that we should use it. I particularly like the extensive docs and general willingness of the author to talk and educate about these issues. At the time I held back because I saw some code that was not clear to me (this) and I didn't want to use code that I do not understand fully. That said, I can definitely be convinced!

The discussion about Secret is possibly slightly more involved as what we have in parity-crypto is sort of a mirror of what we have in ethcore/crypto. The plan is – at least in my head – to move code from there to parity-crypto and @cheme has essentially done that already but it's quite an invasive change so I think it's good to try to do it piece by piece and his approach in that branch is also something I need to review and think through carefully (mostly because I'm slow and ignorant, I don't think there's anything necessarily wrong with it).

[dvdplm]

(So maybe we should split out the secrecy conversation to its own ticket?)

[niklasad1]

/cc @twittner

Any thoughts on zeroize? (I know you implemented something similar in parity-ethereum)

[twittner]

@niklasad1: I can only recommend it. We use it in rust-libp2p and I can think of no reason not to use it here too.