make `randstring` 25% faster for ASCII (#44264)
This uses `_string_n` instead of `StringVector` in order to allocate less,
e.g.:
```
julia> @btime randstring() # master
71.499 ns (2 allocations: 96 bytes)
"DZk2V5Bm"
julia> @btime randstring() # PR
57.832 ns (1 allocation: 32 bytes)
"Hj5PINXU"
```
