[new release] redirect.0.2.1
This PR adds the new release for redirect.0.2.1:
- Fix `with_channel_from_string` for long strings that do not fit in
pipe, by using a thread for outputing the string.
- Some minor optimizations, using the fact that re-raising an exception
preserves the backtrace and the constraint on OCaml version (>=4.03)
ensures that match-with-exception is available.