Use bigstringaf instead of Lwt_bytes to remove unix dependencies

[dinosaure]

Lwt_bytes comes with some syscalls such as read/write on Unix file-descriptor. This PR is a simple replacement of Lwt_bytes by bigstringaf which is compatible with MirageOS.

[aantron]

The PR looks fine.

The CI is showing this warning (as error):

File "src/pure/body.ml", line 139, characters 8-24:
139 |         Bigstringaf.blit !buffer 0 new_buffer 0 !length;
              ^^^^^^^^^^^^^^^^
Error (warning 6): labels src_off, dst_off, len were omitted in the application of this function.

and a few more like it. Could you address these? I will then merge.

[dinosaure]

Yes will do a patch tonight 👍

[dinosaure]

Done ✔️

[aantron]

Thanks!