refactor moonpool_lwt

there's a race condition where, by the time we schedule the
main fiber in `lwt_main`, the event loop is already asleep (maybe
from a previous run). We make sure to wake the loop up.

src/fib/moonpool_fib.ml

@@ -5,8 +5,15 @@
     @since 0.6. *)
 
 module Fiber = Fiber
+[@@deprecated "use picos structured concurrency or something else"]
+
 module Fls = Fls
 module Handle = Handle
+
 module Main = Main
+[@@deprecated "use picos structured concurrency or something else"]
+
+[@@@ocaml.alert "-deprecated"]
+
 include Fiber
 include Main