der: `Reader` and `Writer` traits

[tarcieri]

In the ssh-key crate I've used a similar (but simplified) trait structure inspired by the der crate. It also has Decode/Encode traits, but additionally provides traits for Reader/Writer:

• https://github.com/RustCrypto/formats/blob/22f50ad/ssh-key/src/reader.rs
• https://github.com/RustCrypto/formats/blob/22f50ad/ssh-key/src/writer.rs

These traits are impl'd for types like pem_rfc7468::{Decoder, Encoder}, allowing it to decode from/encode to PEM directly without an intermediate step where data is first decoded from/encoded to a Vec<u8>. This makes it possible to use PEM encoding in heapless environments, which currently isn't possible with the der crate.

PEM decoding is a bit tricky: since the original buffer can't be referenced (since it's encoded as Base64) decoding only works for fully owned types, which happens to be what the ssh-key provides. We don't currently have a way of bounding on such types (though that's trivial to add).

If we were to add this, type signatures would change as follows:

• fn decode(decoder: &mut Decoder<'a>) -> Result<T> => fn decode(decoder: &mut impl Reader<'a>) -> Result<T>
• fn encode(&self, encoder: &mut Encoder<'_>) -> Result<()> => fn encode(&self, encoder: &mut impl Writer) -> Result<()>

We'd also need to come up with new names for the current struct Decoder / struct Encoder, possibly something like SliceDecoder and SliceEncoder.

The encoder trait could also be impl'd for sha2::Sha256 which would allow on-the-fly computation of key fingerprints:

https://github.com/RustCrypto/formats/blob/d42432a3/ssh-key/src/encoder.rs#L107-L113

Presently the spki crate first encodes SubjectPublicKeyInfo to an intermediate buffer before computing a digest of that buffer:

https://github.com/RustCrypto/formats/blob/d42432a3/spki/src/spki.rs#L40-L43

Instead the DER serialization could be computed on the fly and input directly to Sha256 with no intermediate buffer as in the ssh-key crate:

https://github.com/RustCrypto/formats/blob/d42432a3/ssh-key/src/fingerprint.rs#L125-L127

[tarcieri]

The description suggests using &mut impl Reader<'a> and &mut impl Writer but there's a limitation of using impl trait in the argument position: when using a method of a generic type, you presently can't specify generic type arguments as well.

So they should probably be generic types instead.

[tarcieri]

With #635 and #636 this is fully implemented