feat(postgres): Implement Postgres-backed 'advance' function

Transactionally updates a state machine with an incoming event. Note
that this does not yet interpret actions.

finito-postgres/src/error.rs

@@ -5,4 +5,5 @@ use std::result;
 
 pub type Result<T> = result::Result<T, Error>;
 
+#[derive(Debug)]
 pub enum Error { SomeError }