Implement macros for event log creation

[cpitsch]

• Implement From<_> for AttributeValue
• Re-export chrono
• Add macros for event log creation. For instance:

attribute!("concept:name" => "Approve");
attributes!(
    "concept:name" => "Approve",
    "time:timestamp" => chrono::Utc::now()
);

event!("a");
event!("b"; {"org:resource" => "John"});

trace!("a", "b", "c", "d");
trace!(
    {"outcome" => "approved"};
    "a"; {"time:timestamp" => chrono::Utc::now()}, "b", "c", "d"
);

event_log!(
    ["a", "b", "c", "d"],
    ["a", "c", "b", "d"],
);
event_log!(
    {"created_at" => chrono::Utc::now()};
    ["a", "b", "c", "d"] {"approved" => true},
    ["a", "c"; {"org:resource" => "John"}, "b", "d"],
);