Lwt_unix.files_of_directory is very slow

[samoht]

I noticed that Lwt_unix.files_of_directory is always slower that call directly Unix.readdir. Usually it's 2 or 3 times slower, but on some contrived examples I managed to get a 6x slowdown.

I have created https://github.com/samoht/repros/blob/master/readdir.ml to reproduce the issue. Lwt_stream operations seem to make things go slower, as well as the unnecessary allocations done in readdir_n as spotted by @yallop.