Implement `SSL_use_PrivateKey` & `SSL_use_certificate` #86

Job	Run time
Clang Tidy	8s
Clippy	28s
Build+test (stable, ubuntu-latest)	1m 17s
Clippy nightly (optional)	32s
Build+test (beta, ubuntu-latest)	1m 39s
Check for documentation errors	45s
Build+test (nightly, ubuntu-latest)	1m 18s
Format	13s
Miri	1m 1s
Valgrind	1m 16s
	8m 37s

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