Introduce Buf_write.of_flow

[vog]

For buffered serialization into a flow, at the moment there is only Buf_write.with_flow, so those serializers are expected to always run wrapped within a single function.

This PR introduces Buf_write.of_flow which allows a serializer to outlive its invocation, but instead being restricted by a surrounding Switch, as usual. Also, proper closing of the write is ensured by Switch.on_release in success as well as error cases.. This makes Buf_write more straight forward to use e.g. in network protocols, which otherwise need unfortunate workarounds like this one:

• https://github.com/m-click/pgx/blob/upgrade-ocaml5-eio/pgx_eio/src/pgx_eio.ml#L72-L81
• See also: Upgrade to OCaml 5 and EIO arenadotio/pgx#134

Moreover, Buf_write.with_flow is simplified by just calling Buf_write.of_flow within a local Switch.