Implement `SSL_use_PrivateKey` & `SSL_use_certificate` #88

Job	Run time
Clang Tidy	13s
Build+test (stable, ubuntu-latest)	1m 25s
Clippy	27s
Build+test (beta, ubuntu-latest)	1m 12s
Clippy nightly (optional)	37s
Check for documentation errors	42s
Build+test (nightly, ubuntu-latest)	1m 8s
Format	12s
Miri	57s
Valgrind	1m 14s
	8m 7s

Provide feedback