Suspending functions should throw an error when used outside of deflow

[aneilbaboo]

It's very easy in the course of coding to use a suspending function inside a defn. This can lead to Suspend signals being returned as values, which is undefined and ends up being a tricky bug to track down.

Proposal is to deprecate use of ^:suspending metadata and instead implement a macro which registers suspending functions in a dynamic variable accessible to the partitioner.

(def-suspending-fn foo [bar]
   (if bar (listen! ...)))

(foo true) => Exception!!!

(deflow baz [] 
   (foo true) ; suspends
   ...)