Alternative to the other PR. Numbers don't look great?

```
$ cargo run --release -- benchmark -e ~/scratch/bumpalo-arena.so -e ~/scratch/main.so -m perf-counters --stop-after compilation --processes 10 --iterations-per-process 20 --engine-flags="--disable-parallel-compilation --disable-cache" -- benchmarks/spidermonkey/benchmark.wasm benchmarks/bz2/benchmark.wasm benchmarks/pulldown-cmark/benchmark.wasm

compilation :: cache-misses :: benchmarks/bz2/benchmark.wasm

  Δ = 2154.84 ± 788.55 (confidence = 99%)

  main.so is 1.02x to 1.05x faster than bumpalo-arena.so!

  [62141 66843.37 75776] bumpalo-arena.so
  [59523 64688.53 81781] main.so

compilation :: cache-accesses :: benchmarks/pulldown-cmark/benchmark.wasm

  Δ = 8595.08 ± 5795.64 (confidence = 99%)

  bumpalo-arena.so is 1.01x to 1.03x faster than main.so!

  [401717 437779.00 545867] bumpalo-arena.so
  [410908 446374.08 536809] main.so

compilation :: cache-accesses :: benchmarks/bz2/benchmark.wasm

  Δ = 2342.34 ± 2055.41 (confidence = 99%)

  bumpalo-arena.so is 1.00x to 1.03x faster than main.so!

  [133753 143249.76 180756] bumpalo-arena.so
  [135428 145592.10 203005] main.so

compilation :: instructions-retired :: benchmarks/pulldown-cmark/benchmark.wasm

  Δ = 35306.54 ± 32172.66 (confidence = 99%)

  bumpalo-arena.so is 1.00x to 1.01x faster than main.so!

  [8801940 8900728.23 9461825] bumpalo-arena.so
  [8831762 8936034.78 9485351] main.so

compilation :: instructions-retired :: benchmarks/spidermonkey/benchmark.wasm

  Δ = 382102.42 ± 81779.69 (confidence = 99%)

  bumpalo-arena.so is 1.00x to 1.00x faster than main.so!

  [208106667 209068806.32 210101434] bumpalo-arena.so
  [208492210 209450908.75 210448369] main.so

compilation :: cpu-cycles :: benchmarks/bz2/benchmark.wasm

  No difference in performance.

  [2441143 2659275.73 4199423] bumpalo-arena.so
  [2392678 2608641.54 5347877] main.so

compilation :: cpu-cycles :: benchmarks/pulldown-cmark/benchmark.wasm

  No difference in performance.

  [7014995 8113396.99 13667438] bumpalo-arena.so
  [7371875 7996085.34 13293046] main.so

compilation :: cache-misses :: benchmarks/pulldown-cmark/benchmark.wasm

  No difference in performance.

  [131096 167836.79 217530] bumpalo-arena.so
  [128340 169539.35 282605] main.so

compilation :: cache-accesses :: benchmarks/spidermonkey/benchmark.wasm

  No difference in performance.

  [8780974 9800932.96 10522183] bumpalo-arena.so
  [8758505 9771531.34 10590536] main.so

compilation :: cpu-cycles :: benchmarks/spidermonkey/benchmark.wasm

  No difference in performance.

  [185218977 194521807.51 238636300] bumpalo-arena.so
  [181686455 194998321.78 258060749] main.so

compilation :: cache-misses :: benchmarks/spidermonkey/benchmark.wasm

  No difference in performance.

  [4330008 4990485.00 5498381] bumpalo-arena.so
  [4192710 4978995.85 5666416] main.so

compilation :: instructions-retired :: benchmarks/bz2/benchmark.wasm

  No difference in performance.

  [2361496 2431407.85 3123724] bumpalo-arena.so
  [2362101 2429567.66 3057998] main.so
```