make randstring 25% faster for ASCII (JuliaLang#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"
```

stdlib/Random/src/misc.jl

@@ -71,12 +71,20 @@ function randstring end
 
 let b = UInt8['0':'9';'A':'Z';'a':'z']
  global randstring
+
  function randstring(r::AbstractRNG, chars=b, n::Integer=8)
  T = eltype(chars)
- v = T === UInt8 ? Base.StringVector(n) : Vector{T}(undef, n)
- rand!(r, v, chars)
- return String(v)
+ if T === UInt8
+ str = Base._string_n(n)
+ GC.@preserve str rand!(r, UnsafeView(pointer(str), n), chars)
+ return str
+ else
+ v = Vector{T}(undef, n)
+ rand!(r, v, chars)
+ return String(v)
+ end
  end
+
  randstring(r::AbstractRNG, n::Integer) = randstring(r, b, n)
  randstring(chars=b, n::Integer=8) = randstring(default_rng(), chars, n)
  randstring(n::Integer) = randstring(default_rng(), b, n)