Support raw public keys (RFC 7250)

[bifurcation]

Main components here:

• Definition of the client_certificate_type and server_certificate_type extensions
• Related negotiation logic
• Generalization of certificate logic in the handshake to support both raw keys and certificates

[ekr]

Note: this is RFC 7250, not 7520.

Tests seem a bit thin.

[bifurcation]

@ekr This update is substantially cleaned up, should address your comments.