Implement `SSL_get_servername` and `SSL_get_servername_type` #42

Job	Run time
Clippy nightly (optional)	29s
Build+test (stable, ubuntu-latest)	1m 29s
Clang Tidy	9s
Clippy	38s
Build+test (beta, ubuntu-latest)	1m 27s
Check for documentation errors	36s
Build+test (nightly, ubuntu-latest)	1m 24s
Format	7s
Miri	1m 12s
Valgrind	49s
	8m 20s

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