Destructively iterate TreeBuilder fields in TreeBuilder::tree

[jplatte]

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: 2.053.989
peak heap memory consumption: 47,5MB

Benchmark 1: moss -D /mnt/aerynos state build-vfs
  Time (mean ± σ):     358.0 ms ±   2.7 ms    [User: 332.3 ms, System: 25.6 ms]
  Range (min … max):   352.8 ms … 365.2 ms    40 runs