Make it possible to erase all sensitive information after function exit

- What is the feature or code improvement you would like to do in Cranelift? All stack memory and all registers should be clearable when requested by the user of Cranelift.
- What is the value of adding this in Cranelift? This is very useful for cryptographic libraries, where any possible secret leak is one too much.
- Do you have an implementation plan, and/or ideas for data structures or
algorithms to use? https://internals.rust-lang.org/t/annotations-for-zeroing-the-stack-of-sensitive-functions-which-deal-in-transient-secrets/11588/8
- Have you considered alternative implementations? If so, how are they better
or worse than your proposal?