Make vfs::tree::Kind pointer-sized

[jplatte]

By using impossible pointer values of AStr as the representation of the 'Regular' and 'Directory' kinds.

[jplatte]

Now rebased on #655.

moss state build-vfs before:

calls to allocation functions: 2.053.987
peak heap memory consumption: 55,8MB

Benchmark 1: moss -D /mnt/aerynos state build-vfs
  Time (mean ± σ):     360.5 ms ±   2.8 ms    [User: 336.1 ms, System: 24.3 ms]
  Range (min … max):   356.1 ms … 370.6 ms    40 runs

moss state build-vfs after:

calls to allocation functions: 2.053.987
peak heap memory consumption: 52,3MB

Benchmark 1: moss -D /mnt/aerynos state build-vfs
  Time (mean ± σ):     356.5 ms ±   3.9 ms    [User: 333.3 ms, System: 23.1 ms]
  Range (min … max):   350.7 ms … 371.5 ms    40 runs

[jplatte]

Not doing this for now.