Substitute indextree for typed_arena/arena_tree.

[kivikakk]

Keeping the dream alive, by which I mean keeping this rebased in case there's something still useful here. I'll profile it some more in a bit.

See also #616, #617.

[kivikakk]

At the moment it's still a slowdown:

Benchmark 1: ./bench.sh ./comrak-d4fb6782
  Time (mean ± σ):      84.2 ms ±   1.7 ms    [User: 65.1 ms, System: 19.8 ms]
  Range (min … max):    80.7 ms …  88.5 ms    33 runs

Benchmark 2: ./bench.sh ./comrak-main
  Time (mean ± σ):      75.4 ms ±   0.5 ms    [User: 59.4 ms, System: 16.9 ms]
  Range (min … max):    74.1 ms …  76.7 ms    38 runs

Summary
  ./bench.sh ./comrak-main ran
    1.12 ± 0.02 times faster than ./bench.sh ./comrak-d4fb6782

[github-actions]

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
./bench.sh ./comrak-0a4fcc5	142.6 ± 0.9	141.0	144.4	2.43 ± 0.06
./bench.sh ./comrak-main	129.5 ± 1.3	127.4	131.8	2.21 ± 0.05
./bench.sh ./pulldown-cmark	58.7 ± 1.3	57.0	63.6	1.00
./bench.sh ./cmark-gfm	89.3 ± 9.5	81.0	107.1	1.52 ± 0.16
./bench.sh ./markdown-it	259.5 ± 3.3	254.2	265.8	4.42 ± 0.11

Run on Sat Oct 18 11:40:01 UTC 2025