zeroize: stack zeroization support

[tarcieri]

It would be nice to be able to zeroize the stack, particularly being able to track how deep the stack grew while executing a particular section of code, and then zeroing out that whole portion of the stack.

This crate might provide the functionality needed to implement such functionality: https://crates.io/crates/psm

Additionally some proc macro annotation might be needed like #[sensitive] which could inject code to instrument the stack and track its depth.