figure out something for aws_lc_rs's superset of features

[est31]

aws_lc_rs has a superset of the features of ring, but right now we default to using ring's types when both are specified. This was fine until #230, but that PR made it a problem. It hurts the additive nature of cargo's features, ideally the superset of both backends should work.

I see two possible solutions:

1. use aws_lc_rs if both ring and aws_lc_rs are specified. this does not mean that aws_lc_rs should be turned on by default, that is not a good idea. this approach works until ring gets a feature aws_lc_rs doesn't provide. I don't think this is something we need to worry about though (it's unlikely).
2. using both ring and aws_lc_rs if both are specified, ring in the cases where only ring related features are used, aws_lc_rs otherwise. This is manageable for rsa key generation and Support ECDSA_P521_SHA512 when using aws_lc_rs feature #241 , but becomes more complex with more complex features, like entire key pair kinds.

Personally I prefer 1 but would be ok with both. Opinions @djc @cpu?

[djc]

Option 1 sounds okay to me!

[cpu]

I agree that option 1 sounds best here.