Refactor pgp_encrypted_material_t to C++ classes. #3891

Job	Run time
ubuntu-latest [CC gcc; botan; shared libs on; GnuPG stable]	4m 35s
cmake-offline-googletest	4m 7s
cmake-offline-googletest-src	4m 1s
ubuntu-latest [CC clang; botan; shared libs on; GnuPG stable]	4m 12s
ubuntu-latest [CC gcc; openssl; shared libs on; GnuPG stable]	4m 28s
system-sexpp, sexpp shared libs on, rnp shared libs on	3m 8s
ubuntu-latest [CC clang; openssl; shared libs on; GnuPG stable]	3m 55s
package-source	46s
system-sexpp, sexpp shared libs on, rnp shared libs off	2m 49s
ubuntu-20.04 [CC gcc; openssl; shared libs on; GnuPG stable]	4m 1s
system-sexpp, sexpp shared libs off, rnp shared libs on	2m 47s
ubuntu-latest [CC gcc; botan; shared libs off; GnuPG stable]	4m 22s
system-sexpp, sexpp shared libs off, rnp shared libs off	2m 53s
package	1m 9s
debian-tests	13s
	47m 26s

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