hdfs is not compatible with ocaml 5
They use Stream
```
=== ERROR while compiling hdfs.0.4 ===========================================#
context 2.2.0~alpha~dev | linux/x86_64 | ocaml-base-compiler.5.0.0 | file:///home/opam/opam-repository
path ~/.opam/5.0/.opam-switch/build/hdfs.0.4
command ~/.opam/opam-init/hooks/sandbox.sh build ocaml setup.ml -configure --prefix /home/opam/.opam/5.0
exit-code 2
env-file ~/.opam/log/hdfs-7-5514c0.env
output-file ~/.opam/log/hdfs-7-5514c0.out
File "./setup.ml", line 575, characters 4-15:
575 | Stream.from next
^^^^^^^^^^^
Error: Unbound module Stream
```
Moreover hdfs 0.1 uses unsafe string and should have the same upper
bound as hdfs 0.2.
Signed-off-by: Marcello Seri <marcello.seri@gmail.com>