Use `X509_STORE` for root certificate storage #178

Job	Run time
Format	20s
Clippy	36s
Build+test (stable, ubuntu-latest)	1m 40s
Clang Tidy	8s
Build+test (beta, ubuntu-latest)	2m 5s
Clippy nightly (optional)	32s
Check for documentation errors	42s
Build+test (nightly, ubuntu-latest)	1m 44s
Miri	1m 1s
Packaging	2m 55s
Valgrind	2m 10s
	13m 53s

Provide feedback