astr: Use ThinArc

[jplatte]

Reduces memory usage without impacting performance of my simple vfs-building benchmark.

Before:

calls to allocation functions: 3.000.710
peak heap memory consumption: 74,7MB

Benchmark 1: moss -D /mnt/aerynos state build-vfs
  Time (mean ± σ):     411.1 ms ±   2.2 ms    [User: 381.8 ms, System: 27.0 ms]
  Range (min … max):   407.3 ms … 417.8 ms    40 runs

After:

calls to allocation functions: 3.000.710
peak heap memory consumption: 62,7MB

Benchmark 1: moss -D /mnt/aerynos state build-vfs
  Time (mean ± σ):     412.3 ms ±   2.9 ms    [User: 385.9 ms, System: 26.3 ms]
  Range (min … max):   408.5 ms … 420.9 ms    40 runs

[tarkah]

Lgtm. Would love some benchmarking numbers posted before we merge.