SmolStr has an Arc internally for large strings. This allows
cheap copies of large strings, but we lose that ability
when we convert the SmolStr to a &str and then reconstruct a
SmolStr from that slice.
I was hoping for some larger gains here, considering the impact
of this code change, but it only removes ~50k allocations,
while the impact on the runtime is not noticeable at all.
Still, I believe this is the right thing to do.
Before:
```
allocations: 2338981
Time (mean ± σ): 988.3 ms ± 17.9 ms [User: 690.2 ms, System: 206.4 ms]
Range (min … max): 956.4 ms … 1016.3 ms 10 runs
```
After:
```
allocations: 2287723
Time (mean ± σ): 989.8 ms ± 23.2 ms [User: 699.2 ms, System: 197.6 ms]
Range (min … max): 945.3 ms … 1021.4 ms 10 runs
```
