Clearing sensitive data before exiting the code

[mkaihara]

Please make sure that sensitive data, such as private keys, that are stored in temporary variables are cleared before exiting the function. It is not safe to just leave it out of scope as it may be exploited with attacks. Also make sure that the function that clears the data is not skipped by optimization of the compiler.

[burdges]

I'd wait for https://github.com/rust-lang/rfcs/pull/2859 and see how dalek, etc. use it before worrying about this.