An equivalent to OPENSSL_cleanse or something to zeroize OpenSSL keys

[JosePisco]

Working with PKey types, it can be of interest to have tools that clear the key buffers on drop, something like the Zeroize crate would do but for OpenSSL types.

I believe the OpenSSL library offers the OPENSSL_cleanse tool that writes zeroes on the buffer pointed to by the first argument.

[botovq]

EVP_PKEYs are dropped with EVP_PKEY_free which should use the appropriate cipher-internal freeing function which in turn should cleanse all private data unless there is a a bug in the underlying OpenSSL library that you're using. Auditing this for the key types you care about is going to be a bit of work but this should not need extra handling by rust-openssl or its users.