Use `X509_STORE` for root certificate storage #176

Job	Run time
Clippy nightly (optional)	33s
Clang Tidy	11s
Build+test (stable, ubuntu-latest)	1m 39s
Clippy	32s
Build+test (beta, ubuntu-latest)	1m 50s
Check for documentation errors	1m 6s
Build+test (nightly, ubuntu-latest)	1m 39s
Format	18s
Miri	1m 14s
Packaging	2m 44s
Valgrind	2m 10s
	13m 56s

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