[Goldloli]

Fixes #407

Summary

Each Localization used to allocate an ~8 KB block the first time it formatted a message, because the bundle cache (Cache/AsyncCache in fluent-fallback) stores bundles in a ChunkyVec whose first chunk reserves DEFAULT_CAPACITY = 32 elements (~6.4 KB requested for a ~200 B bundle). A Localization normally caches one bundle (en-US) or two (locale + en-US fallback), so most of that block is never used.

This stores each cached bundle behind its own Box in a plain Vec:

• References handed out by Cache::get/Cache::push_get stay valid regardless of how the Vec grows, because a Box never moves its contents — this keeps the pin-safety guarantee that motivated ChunkyVec, addressing the caveat in the third option of the issue.
• The cache now grows one element at a time, so the fixed cost for one or two bundles is a couple of small allocations instead of one 8 KB block.
• The chunky-vec dependency is dropped (options 1 and 2 would have required changes to that crate, whose API has no way to pick a chunk capacity).

Verification

• cargo test -p fluent-fallback — 3 unit + 9 integration + 2 doc tests pass.
  • New caching_few_bundles_avoids_chunk_sized_allocations counts allocations in the 6–7 KB band while caching two bundle-sized (200 B) elements via a #[global_allocator]: it fails on main with exactly one 6400-byte request (32 × 200) and passes with this change; run 10× without flakes.
  • New references_stay_stable_across_growth keeps references from push_get across further growth and reads them afterwards, pinning the address-stability guarantee.
• cargo test --workspace — pass.
• cargo fmt --all -- --check — clean.
• cargo clippy --all-features -- -Dwarnings — no new warnings from this change (cache.rs is clean; the remaining must_use error from async_trait on current stable reproduces identically on main).