Implement UV-workflow for CTAP 2.1

[msirringhaus]

First draft of CTAP2.1 UV.

Significant conceptual changes:

• Instead of letting the consumer of this crate decide, which PIN-errors are recoverable and which are not, we do that now internally. And only send a Sender<Pin> for PinRequired and InvalidPin. All other PIN-errors get send "one-way" as a normal status update.
• For this, we need to repackage PinErrors yet again into a different enum
• Handle uv = "discouraged" somewhat (although not pretty, and Firefox doesn't use it yet)
• get_shared_secret() doesn't need any input anymore and also does not return the AuthenticatorInfo. For this get_authenticator_info() is now always unpacked with the assumption that it exists and we return a DeviceNotInitialized-error otherwise.

This needs rebasing once #239 lands (and may influence how #239 will look like).

[jschanck]

Let's rebase this on top of #239 and work out any further details here---I'll close #239 once you do that.