WIP: add test for mixed DER/BER encoding CMS

[smndtrl]

As requested in #813 a test for mixed encoding CMS.

@tarcieri I didn't find my original BER file from the Apple MDM Signature so I just checked the example at https://lapo.it/asn1js/# and it's very similar in that it starts with an indefinite length tag, uses multiple along the way and also uses constructed undefined length OctetStrings.

Both the DER/BER file have the same e_content. Currently the DER encoded file is used in the test and works fine.

[tarcieri]

Suggested change

	let path = "./tests/examples/cms_der.bin";
	let bytes = fs::read(&path).expect(&format!("Failed to read from {}", &path));
	let content = ContentInfo::from_der(&bytes).expect("expected valid data");
	match content {
	ContentInfo::SignedData(Some(data)) => {
	assert_eq!(
	data.encap_content_info
	.e_content
	.unwrap()
	.decode_into::<OctetStringRef>()
	.unwrap()
	.as_bytes()
	.len(),
	10034
	)
	}
	_ => panic!("expected ContentInfo::SignedData(Some(_))"),
	}
	let bytes = include_bytes!("examples/cms_der.bin");
	let content = match ContentInfo::from_der(bytes) {
	Ok(ContentInfo::SignedData(Some(data))) => data,
	other => panic!("unexpected result: {:?}", other),
	};
	assert_eq!(
	content
	.encap_content_info
	.e_content
	.unwrap()
	.decode_into::<OctetStringRef>()
	.unwrap()
	.as_bytes()
	.len(),
	10034
	);

[tarcieri]

Looking at the BER example, it looks like it's using indefinite lengths fairly pervasively for several (nested) fields.

Making this work will probably involve modifying custom derive to be able to generate Decode impls for BER-tolerant SEQUENCEs.

[tarcieri]

Here's a stepping stone towards a solution: #830

[tarcieri]

Merged in #840