This will allow cohttp-eio to use a different type in future, which
should make streaming easier.

Makes next commit easier to read:

- Use `respond` helpers in example. Avoids it having to change.
- Remove useless `IO.t` and `>>=`.
- Move some functions later in the file, and they'll need to use
  `write`.

This changes the response type to `writer -> unit`. This allows
handlers to write the response inside the function, rather than
returning a request to cohttp to write it later. That's useful because
it allows e.g. streaming from an open file and then closing it
afterwards.

Partial application means that code using `respond_string` etc will
continue to work as before.

This also exposes a more polymorphic version of the `respond` function
that accepts sub-types of `Flow.source`, so that callers don't need to
cast the body.

Co-authored-by: Marcello Seri <mseri@users.noreply.github.com>