Use `X509_STORE` for root certificate storage #179

Job	Run time
Valgrind	2m 7s
Clang Tidy	11s
Build+test (stable, ubuntu-latest)	1m 44s
Clippy	34s
Build+test (beta, ubuntu-latest)	1m 39s
Clippy nightly (optional)	32s
Check for documentation errors	39s
Build+test (nightly, ubuntu-latest)	1m 45s
Format	16s
Miri	1m 10s
Packaging	2m 36s
	13m 13s

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