Store durations in a tree

Recorded durations are stored in a tree structure instead of a list
making it possible to trace the origin of each entry.
Rename benchmark-init/show-durations to benchmark-init/show-durations-tabulated

This separates the tabulated list mode from the new tree mode.
Tree presentation mode

A new mode for displaying the results as a tree of require/load
operations making it easy to trace their origins.