slice time between children

[hjdivad]

if parent a has children b, c we currently report three times

• a selftime
• b selftime
• c selftime

but it is often interesting to know time (and other stats) of a separated between (0-b), (b-c), (c-end).

Seems like this could be done by utilizing a fix for #13

[hjdivad]

It's worth noting that merely doing #15 handles this for times.

To get it for counters as well, we may want to do something like

• leaf nodes only have counters
• when a leaf node becomes an intermediate node, do a conversion that shifts the counters into a first child.

eg

heimdall.start('a');
heimdall.statsFor('x').foo++;  // a has stats
heimdall.start('b');
// at this point a no longer has stats, but has two children, a_0 who starts at a start and stops at b start.  a_0 has the stats that a used to have.

heimdall.stop('b');
// at this point there is now a_1 who starts when b stops and will stop when a stops
heimdall.statsFor('x').foo++;
heimdall.stop('a');

[runspired]

This landed as 0.3