Use `X509_STORE` for root certificate storage #177

Job	Run time
Clippy nightly (optional)	34s
Clippy	32s
Clang Tidy	6s
Build+test (stable, ubuntu-latest)	1m 44s
Check for documentation errors	43s
Build+test (beta, ubuntu-latest)	1m 42s
Format	16s
Build+test (nightly, ubuntu-latest)	1m 40s
Miri	1m 8s
Packaging	2m 58s
Valgrind	2m 19s
	13m 42s

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