Refactor pgp_encrypted_material_t to C++ classes. #3881

Job	Run time
ubuntu-latest [CC gcc; botan; shared libs on; GnuPG stable]	4m 10s
cmake-offline-googletest-src	3m 58s
ubuntu-latest [CC clang; botan; shared libs on; GnuPG stable]	4m 19s
cmake-offline-googletest	4m 15s
ubuntu-latest [CC gcc; openssl; shared libs on; GnuPG stable]	4m 14s
ubuntu-latest [CC clang; openssl; shared libs on; GnuPG stable]	4m 4s
system-sexpp, sexpp shared libs on, rnp shared libs on	2m 47s
package-source	29s
ubuntu-20.04 [CC gcc; openssl; shared libs on; GnuPG stable]	3m 50s
system-sexpp, sexpp shared libs on, rnp shared libs off	3m 27s
ubuntu-latest [CC gcc; botan; shared libs off; GnuPG stable]	4m 38s
system-sexpp, sexpp shared libs off, rnp shared libs on	3m 6s
system-sexpp, sexpp shared libs off, rnp shared libs off	2m 55s
package	55s
debian-tests	12s
	47m 19s

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