pkcs8: better `no_alloc` support for `DecodePrivateKey`/`EncodePrivateKey` traits

[tarcieri]

I am punting on these for the forthcoming v0.11 release, but the next time we do make breaking changes it would be nice to get them resolved:

• PEM decoding: currently the whole pem feature enables alloc, but it would be nice to support no_alloc decoding by way of the DecodePrivateKey trait, especially as the der crate is capable of on-the-fly PEM decoding
• Encoding: currently the whole EncodePrivateKey trait is gated on alloc, but it would be nice to support a no_alloc encoder for both DER and PEM that can write into a supplied buffer and return a slice

[dishmaker]

For anyone wondering how PEM can decode to a no_alloc, here's a p256 example (DER, not PEM):

let der_key = p256::SecretKey::from_pkcs8_der(&PKCS8_PRIVATE_KEY_DER[..]).unwrap();

https://github.com/RustCrypto/elliptic-curves/blob/82c493eda1cb8b751ddf970368ecec1a592df88c/p256/tests/pkcs8.rs#L49-L54

[tarcieri]

That's a DER example, which is supported via these traits in no_alloc use cases.

We support on-the-fly PEM decoding, which is encoded as text and begins with BEGIN PRIVATE KEY.

However it only works in no_alloc scenarios if decoding to a stack-allocated data structures that don't use references.