Implements Tracing without Performance (spec).

This is done by having an additional PropagationContext attribute on the scope that holds a "fake" trace_id and span_id.
When applying the scope to an event, if there is no ongoing span set on the scope, we create the trace context from the PropagationContext.
Additionally, adds an API scope.iter_trace_propagation_headers that should be preferred over something like:

sentry::configure_scope(|scope| {
	if let Some(span) = scope.get_span() {
		headers = Some(span.iter_headers());
	} 
});

because it will use the PropagationContext as a fallback.

Closes #796

Also makes a minor adjustment to trace and span ids where we now use the Display impl for Debug, otherwise we were getting u8 arrays when using e.g. dbg! with the derived one which is not very useful.