This changes the way we print things to first construct a mapping from
events to the children and uses that mapping to actually print things.
It should not change the actual output that we produce.
The new approach two benefits:
* It avoids a potential quadratic behavior of the previous approach.
For instance, for a vector of N elements:
```
[Message{level: (N - 1)}, ..., Message{level: 1}, Message{level: 0}]
```
we would first do a linear scan to find entry with level 0, then
another scan to find one with level 1, etc.
* It makes it much easier to improve the output in the future, because
we now pre-compute the children for each entry and can easily take
that into account when printing.
Signed-off-by: Michal Terepeta <michal.terepeta@gmail.com>
39cbb6b620