radixsort much slower than buit-in sort (regarding string sorting)

```julia
lines = open("f.txt") do file
    readlines(file) #has approx 4.3m lines
end
@time sort(lines)
@time radixsort(lines)
```
delivers the following output:
```
2.585242 seconds (9 allocations: 49.539 MiB, 0.42% gc time)
18.793147 seconds (14.83 k allocations: 2.916 GiB, 36.62% gc time)
```