Add {X509Req, X509}::check_private_key

[alebastr]

• BoringSSL and AWS-LC marked arguments as const for about a year, which I assume is enough to apply it unconditionally with always-use-git-master kind of libraries.
• Even before constification, the OpenSSL implementation did not mutate the objects in any way. I hope that justifes the use of shared refs.
• The interface is intentionally limited to PKeyRef<Private>, as the man page admits that accepting public keys in the underlying functions is a bug.