Add various missing functionality

[dewyatt]

I took a look at what new functionality in rnp was not yet exposed in ruby-rnp and added support for:

• s2k iteration calculation
• enable/disable debugging
• guess_contents
• check feature support
• list supported features
• key revocation checks
• key and uid signature support
• key creation, expiration, and primary_grip accessors
• Output#write
• import_keys