As aluded to in the previous commit. This approach keeps track of hard
links information much the same way as ncdu 1.16, with the main
difference being that the actual /counting/ of hard link sizes is
deferred until the scan is complete, thus allowing the use of a more
efficient algorithm and amortizing the counting costs.

As an additional benefit, the links listing in the information window
now doesn't need a full scan through the in-memory tree anymore.

A few memory usage benchmarks:

              1.16  2.0-beta1  this commit
root:          429        162          164
backup:       3969       1686         1601
many links:    155        194          106
many links2*:  155        602          106

(I'm surprised my backup dir had enough hard links for this to be an
improvement)
(* this is the same as the "many links" benchmarks, but with a few
parent directories added to increase the tree depth. 2.0-beta1 doesn't
like that at all)

Performance-wise, refresh and delete operations can still be improved a
bit.