Add CodSpeed benchmark suite

[charliermarsh]

This adds a repeatable performance suite for the design work in the other draft PRs. The benchmarks are separated by concern: collection covers move-aware collection, mutation covers reserve and append paths, and heap_layout covers construction, clone/drop, and traversal workloads around both the 12-byte and 16-byte representation boundaries.

[codspeed-hq]

Congrats! CodSpeed is installed 🎉

🆕 144 new benchmarks were detected.

You will start to see performance impacts in the reports once the benchmarks are run from your default branch.

Detected benchmarks

• lean_empty[0] (Memory): 0 B
• lean_empty[1] (Memory): 0 B
• lean_empty[2] (Memory): 0 B
• lean_empty[64] (Memory): 0 B
• lean_empty[8] (Memory): 0 B
• lean_heap[0] (Memory): 0 B
• lean_heap[1] (Memory): 0 B
• lean_heap[2] (Memory): 80 B
• lean_heap[64] (Memory): 3 KB
• lean_heap[8] (Memory): 304 B
• lean_inline[0] (Memory): 0 B
• lean_inline[1] (Memory): 0 B
• lean_inline[2] (Memory): 0 B
• lean_inline[64] (Memory): 772 B
• lean_inline[8] (Memory): 88 B
• lean_reserved_first[0] (Memory): 0 B
• lean_reserved_first[1] (Memory): 0 B
• lean_reserved_first[2] (Memory): 0 B
• lean_reserved_first[64] (Memory): 0 B
• lean_reserved_first[8] (Memory): 0 B
• ...

ℹ️ Only the first 20 benchmarks are displayed. Go to the app to view all benchmarks.

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