Use StringMemory instead of StringVector where possible (#53962)
On `1.11.0-alpha2`
Old:
```julia
@benchmark Base.dec($0x1, $0, $false)
BenchmarkTools.Trial: 10000 samples with 994 evaluations.
Range (min … max): 33.702 ns … 4.242 μs ┊ GC (min … max): 0.00% … 97.61%
Time (median): 37.626 ns ┊ GC (median): 0.00%
Time (mean ± σ): 45.787 ns ± 147.794 ns ┊ GC (mean ± σ): 14.53% ± 4.47%
▄▅▆▇█▇▇▅▃▃▂▂▂▁ ▁▂▁▁▁ ▁▁ ▁ ▂
▄▇███████████████▇▇██████▇█▆▆▄▄▃▄▅▄▆▇████████▆▅▅▇▆▅▆▄▄▅▄▄▄▁▅ █
33.7 ns Histogram: log(frequency) by time 67.5 ns <
Memory estimate: 88 bytes, allocs estimate: 3.
```
New:
```julia
BenchmarkTools.Trial: 10000 samples with 995 evaluations.
Range (min … max): 27.538 ns … 3.397 μs ┊ GC (min … max): 0.00% … 97.86%
Time (median): 30.151 ns ┊ GC (median): 0.00%
Time (mean ± σ): 34.547 ns ± 105.101 ns ┊ GC (mean ± σ): 10.37% ± 3.39%
▁ █▆▃ ▁
▂▂▃▃▅█████▆████▆▄▄▃▃▃▃▃▃▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▁▂▂▂▂▂▁▂▂▂▂▂▂▂▂▂▂▂▂▂ ▃
27.5 ns Histogram: frequency by time 43.8 ns <
Memory estimate: 56 bytes, allocs estimate: 2.
```
Fixes #53950, actually now even faster than `1.10.2`.
It doesn't look like the length is ever changed and we don't return
these `StringMemory`s so this change should be fine.
