helpers: Make UniqueStringsReuse allocation free

helpers: Make UniqueStringsReuse allocation free

This should be faster for all of our use cases (small string slices), but more important, it makes UniqueStringsReuse allocation free.

```bash
name                                 old time/op    new time/op    delta
UniqueStrings/Safe-16                   776ns ± 0%     305ns ± 0%   -60.68%  (p=0.029 n=4+4)
UniqueStrings/Reuse_slice-16            621ns ± 3%     174ns ± 0%   -72.01%  (p=0.029 n=4+4)
UniqueStrings/Reuse_slice_sorted-16     319ns ± 4%     313ns ± 0%    -2.01%  (p=0.029 n=4+4)

name                                 old alloc/op   new alloc/op   delta
UniqueStrings/Safe-16                    224B ± 0%      128B ± 0%   -42.86%  (p=0.029 n=4+4)
UniqueStrings/Reuse_slice-16            96.0B ± 0%      0.0B       -100.00%  (p=0.029 n=4+4)
UniqueStrings/Reuse_slice_sorted-16     24.0B ± 0%     24.0B ± 0%      ~     (all equal)

name                                 old allocs/op  new allocs/op  delta
UniqueStrings/Safe-16                    7.00 ± 0%      1.00 ± 0%   -85.71%  (p=0.029 n=4+4)
UniqueStrings/Reuse_slice-16             6.00 ± 0%      0.00       -100.00%  (p=0.029 n=4+4)
UniqueStrings/Reuse_slice_sorted-16      1.00 ± 0%      1.00 ± 0%      ~     (all equal)
```