zeroize: `libc` feature?

[tarcieri]

It looks like on several operating systems, the libc crate has wrappers for secure memset/bzero primitives:

rust-lang/libc#2156

We could potentially have an optional dependency on libc which calls into these primitives from e.g. volatile_set, while defaulting to a portable implementation. Many programs are already linking with libc anyway.

We would somewhat annoyingly have to build an abstraction across them.

[newpavlov]

IMO unless there is a practical demonstration of the asm! approach not working, I don't think we should bother with it.