A fairly large change that replaces Zaps in the codebase with "Zapping"
which is a tagged union consisting of a resolved Zap and a Pending Zap.
These are both counted as Zaps everywhere in Damus, except pending zaps
can be cancelled (most of the time).
Closes#5
This adds encrypted direct message support to damus
Changelog-Added: Implement NIP04: Encrypted Direct Messages
Signed-off-by: William Casarin <jb55@jb55.com>