Consider making *ring* dependency optional

[ctz]

The ultimate goal here would be to allow use of the webpki crate with other cryptography libraries.

This would come in two parts, I think:

• replacing remaining references to ring::io -- much of this was already done, but a handful of instances remain.
• rework SignatureAlgorithm to be a trait that can be implemented outside the crate, with implementations backed by ring being provided by default (eg, ECDSA_P256_SHA256). Alongside, we should export AlgorithmIdentifier and the common values thereof.

[djc]

Could/should we share a signature trait with rustls somehow?