pem-rfc7468: add Decoder struct

[tarcieri]

This adds a Decoder struct which supports one configurable setting: the Base64 line-wrapping width.

From RFC7468 Section 2:

Parsers MAY handle other line sizes.

Closes #176

[tarcieri]

@kjvalencik this is what I propose for addressing #176. It would allow you to do:

pem_rfc7468::Decoder { wrap_width: 70 }.decode_vec(input)

...or for a fully futureproofed version:

pem_rfc7468::Decoder { wrap_width: 70, ..Default::default() }.decode_vec(input)

[kjvalencik]

@tarcieri I really like this API with the Decoder. Unfortunately, this only works when line-length is at a 4:3 ratio, otherwise base64 decode fails.

For example, this works with 64 and 72. But, if I try to decode with 70 (what OpenSSH uses), it fails to decode base64 (since it's incomplete input).

    #[test]
    fn openssh_example() {
        let pem = include_bytes!("../tests/examples/openssh.pem");
        let result = Encapsulation::parse(pem).unwrap();
        assert_eq!(result.label, "OPENSSH PRIVATE KEY");

        let decoder = Decoder { wrap_width: 70 };
        let mut buf = [0; 2048];
        result.decode(&decoder, &mut buf).unwrap();
    }

---- decoder::tests::openssh_example stdout ----
thread 'decoder::tests::openssh_example' panicked at 'called `Result::unwrap()` on an `Err` value: Base64', pem-rfc7468/src/decoder.rs:308:43

[tarcieri]

Aah wow, that's really annoying. Supporting anything else would require a buffered Base64 decoder, which isn't exactly trivial to implement, and something I'd like to avoid in proper PKCS/PKIX use cases.

I suppose I could add an error case for if wrap_width % 4 != 0 for now, and if someone would like to contribute a buffered decoder, it could eventually just work.