Refactor pgp_encrypted_material_t to C++ classes. #3895

Job	Run time
ubuntu-latest [CC gcc; botan; shared libs on; GnuPG stable]	5m 22s
ubuntu-latest [CC clang; botan; shared libs on; GnuPG stable]	4m 36s
ubuntu-latest [CC gcc; openssl; shared libs on; GnuPG stable]	4m 25s
ubuntu-latest [CC clang; openssl; shared libs on; GnuPG stable]	4m 24s
ubuntu-20.04 [CC gcc; openssl; shared libs on; GnuPG stable]	3m 55s
ubuntu-latest [CC gcc; botan; shared libs off; GnuPG stable]	4m 59s
cmake-offline-googletest	4m 30s
cmake-offline-googletest-src	4m 22s
system-sexpp, sexpp shared libs on, rnp shared libs on	3m 35s
package-source	1m 9s
system-sexpp, sexpp shared libs on, rnp shared libs off	3m 26s
system-sexpp, sexpp shared libs off, rnp shared libs on	3m 29s
system-sexpp, sexpp shared libs off, rnp shared libs off	3m 32s
package	54s
debian-tests	9s
	52m 47s

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