Implement `SSL_get_servername` and `SSL_get_servername_type` #41

Job	Run time
Clippy nightly (optional)	46s
Clippy	29s
Build+test (stable, ubuntu-latest)	1m 22s
Clang Tidy	11s
Check for documentation errors	57s
Build+test (beta, ubuntu-latest)	2m 14s
Build+test (nightly, ubuntu-latest)	1m 23s
Miri	1m 19s
Format	14s
Valgrind	1m 2s
	9m 57s

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