Refactor rnp_key_store_t to rnp::KeyStore. #3165

Job	Run time
ubuntu-latest [CC gcc; botan; shared libs on; GnuPG stable]	7m 22s
ubuntu-latest [CC clang; botan; shared libs on; GnuPG stable]	7m 4s
ubuntu-latest [CC gcc; openssl; shared libs on; GnuPG stable]	8m 54s
ubuntu-latest [CC clang; openssl; shared libs on; GnuPG stable]	6m 58s
ubuntu-20.04 [CC gcc; openssl; shared libs on; GnuPG stable]	6m 53s
ubuntu-latest [CC gcc; botan; shared libs off; GnuPG stable]	7m 20s
cmake-offline-googletest	7m 33s
cmake-offline-googletest-src	8m 8s
system-sexpp, sexpp shared libs on, rnp shared libs on	5m 39s
system-sexpp, sexpp shared libs on, rnp shared libs off	6m 24s
system-sexpp, sexpp shared libs off, rnp shared libs on	4m 40s
system-sexpp, sexpp shared libs off, rnp shared libs off	6m 3s
package-source	31s
package	1m 2s
debian-tests	14s
	1h 24m 45s

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