src: do not shrink async resources on every pop

[nigrosimone]

AsyncHooks::pop_async_context() calls shrink_to_fit() on native_execution_async_resources_ after every pop. With the MSVC and libc++ standard libraries, shrink_to_fit() on an empty std::deque frees its storage, so every top-level callback scope allocates it again on push and frees it on pop. libstdc++ does nothing here, so on Linux nothing change.

Now the deque keeps its storage, like the async_ids_stack_ next to it, which never shrinks.

Until v24 the shrink ran only when the size was over 16 and under half of the capacity, so only after a deep nesting. Since #56457 it runs on every pop.

The same pattern in a small program, on Window with MSVC: 136-147 ns per callback with shrink_to_fit(), 6 ns without. macOS uses libc++, which frees the empty deque in the same way (not measured).

#include <chrono>
#include <cstdio>
#include <deque>
#include <variant>

std::deque<std::variant<int*, long*>> resources;
int value;

// Same as push_async_context() and pop_async_context() at depth 0.
__declspec(noinline) void Push() { resources.resize(1); resources[0] = &value; }
__declspec(noinline) void Pop() { resources.resize(0); resources.shrink_to_fit(); }

int main() {
  const int n = 5000000;
  auto start = std::chrono::steady_clock::now();
  for (int i = 0; i < n; i++) { Push(); Pop(); }
  auto end = std::chrono::steady_clock::now();
  printf("%.1f ns\n", std::chrono::duration<double, std::nano>(end - start).count() / n);
}

Refs: #66316
Refs: nodejs/performance#24

Disclosure: I used Opus 5.5 (Max) as coding assistant

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.37%. Comparing base (66f26d3) to head (a3a1b3c).
⚠️ Report is 57 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #66344      +/-   ##
==========================================
- Coverage   90.38%   90.37%   -0.02%     
==========================================
  Files         790      792       +2     
  Lines      274497   275445     +948     
  Branches    52557    52781     +224     
==========================================
+ Hits       248100   248922     +822     
- Misses      16879    16936      +57     
- Partials     9518     9587      +69     

Files with missing lines	Coverage Δ
src/env.cc	82.08% <ø> (-0.20%)	⬇️

... and 66 files with indirect coverage changes

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