make randstring 25% faster for ASCII

[rfourquet]

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"

[KristofferC]

Could use a rebase it seems.

[vtjnash]

Random.UnsafeView perhaps really should have been deprecated a long time ago.

[rfourquet]

Random.UnsafeView perhaps really should have been deprecated a long time ago.

There is an issue for that: #36718
But this PR is an example where it still proves to be useful, unless I'm missing another way. As mentioned in the issue though, we could definitely try to make UnsafeView hold a reference to the parent and see if performance doesn't get worse (although that would mean compiling more methods, for each parent type, instead of for each eltype).

[LilithHafner]

This looks good to me. I think that qualms about UnsafeView are valid but don't block this PR.