First, we switch from a `BTreeSet` to a `HashSet` because clearing a `BTreeSet`
will deallocate the btree's nodes but clearing a `HashSet` will not deallocate
the backing hash table, saving the space to reuse for future insertions.
Then, we reuse the same set (and therefore the same allocation) across every
call to `can_optimize_var_lookup`.
This results in a 1.22x to 1.32x speed up on various Sightglass benchmarks:
```
compilation :: nanoseconds :: benchmarks/pulldown-cmark/benchmark.wasm
Δ = 39478181.76 ± 3441880.32 (confidence = 99%)
main.so is 0.75x to 0.79x faster than reuse-set.so!
reuse-set.so is 1.27x to 1.32x faster than main.so!
[160128343 172174751.09 213325968] main.so
[115055695 132696569.33 200782128] reuse-set.so
compilation :: nanoseconds :: benchmarks/bz2/benchmark.wasm
Δ = 22576954.88 ± 1830771.68 (confidence = 99%)
main.so is 0.77x to 0.81x faster than reuse-set.so!
reuse-set.so is 1.25x to 1.29x faster than main.so!
[100449245 106820149.65 118628066] main.so
[77039172 84243194.77 128168647] reuse-set.so
compilation :: nanoseconds :: benchmarks/spidermonkey/benchmark.wasm
Δ = 664533554.97 ± 22109170.05 (confidence = 99%)
main.so is 0.81x to 0.82x faster than reuse-set.so!
reuse-set.so is 1.22x to 1.23x faster than main.so!
[3549762523 3640587103.35 3798662501] main.so
[2793335181 2976053548.38 3192950484] reuse-set.so
```
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