libexpr: plug ExprCall memory leak

[Radvendii]

This PR does a similar transformation as #14539, but to ExprCall rather than ExprAttrs. It moves the storage into the arena after parsing by using std::optional<std::pmr::vector<Expr *>> instead of std::vector<Expr *>.

I verified that this does plug the memory leak using ASAN.

Unlike #14539, which slowed parsing down a bit, this seems to come with a tiny speed up. Not sure why it's affecting speed differently.

Performance numbers

$ poop -d 30000 '../master/result/bin/nix-instantiate --parse ../../nixpkgs/pkgs/development/haskell-modules/hackage-packages.nix' '../feature/result/bin/nix-instantiate --parse ../../nixpkgs/pkgs/development/haskell-modules/hackage-packages.nix'
Benchmark 1 (85 runs): ../master/result/bin/nix-instantiate --parse ../../nixpkgs/pkgs/development/haskell-modules/hackage-packages.nix
  measurement          mean ± σ            min … max           outliers         delta
  wall_time           353ms ± 3.82ms     347ms …  370ms          3 ( 4%)        0%
  peak_rss            108MB ±  215KB     108MB …  109MB          0 ( 0%)        0%
  cpu_cycles         1.31G  ± 14.4M     1.29G  … 1.39G           3 ( 4%)        0%
  instructions       3.70G  ± 8.76K     3.70G  … 3.70G           1 ( 1%)        0%
  cache_references   29.1M  ± 1.05M     28.4M  … 35.0M          12 (14%)        0%
  cache_misses       2.10M  ± 12.5K     2.07M  … 2.14M           3 ( 4%)        0%
  branch_misses      3.94M  ± 26.3K     3.91M  … 4.06M           3 ( 4%)        0%
Benchmark 2 (87 runs): ../feature/result/bin/nix-instantiate --parse ../../nixpkgs/pkgs/development/haskell-modules/hackage-packages.nix
  measurement          mean ± σ            min … max           outliers         delta
  wall_time           345ms ± 8.08ms     336ms …  398ms          9 (10%)        ⚡-  2.2% ±  0.5%
  peak_rss            109MB ±  212KB     108MB …  109MB          0 ( 0%)          +  0.9% ±  0.1%
  cpu_cycles         1.28G  ± 35.1M     1.24G  … 1.52G           9 (10%)        ⚡-  2.5% ±  0.6%
  instructions       3.68G  ± 8.80K     3.68G  … 3.68G           0 ( 0%)          -  0.5% ±  0.0%
  cache_references   29.9M  ± 3.41M     28.5M  … 57.6M           9 (10%)          +  2.6% ±  2.6%
  cache_misses       2.10M  ± 25.8K     2.06M  … 2.22M           6 ( 7%)          +  0.1% ±  0.3%
  branch_misses      4.02M  ±  102K     3.96M  … 4.54M           8 ( 9%)        💩+  2.2% ±  0.6%

Motivation

Plugging memory leaks will enable us to fuzz the parser. See the tracking issue for big-picture motivation.

Context

• Tracking issue: Managed Heap tracking issue #14088
• Similar transformation for ExprAttrs: libexpr: move ExprAttrs data into Exprs::alloc (take 2) #14539

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