`ds/start` hangs indefinitely consuming a `promesa.exec.csp/chan`

[Ramblurr]

The start function hands indefinitely when a component returns a map containing a promesa channel.

Promesa channels act like seqs apparently?

To reproduce:

1. Create a component that returns {:a-chan (sp/chan)} from its ::ds/start function. Where sp/chan is https://funcool.github.io/promesa/latest/promesa.exec.csp.html#var-chan
2. Create another component that references the first one using a local-ref (with the goal of using :a-chan inside it)
3. Start the donut system

  (require '[donut.system :as ds])
  (require '[promesa.exec.csp :as sp])
  (ds/start
   {::ds/defs {:group {:first {::ds/start (fn [_]
                                            {:a-chan (sp/chan)})}

                       :second {::ds/start  (fn [{::ds/keys [config]}]
                                              {:got config})
                                ::ds/config {:first [::ds/local-ref [:first]]}}}}})
;; Warning: evaling this will hang

Result: The eval of ds/start will never return, as it enters the sp/chan->seq function and blocks waiting for something to appear on the channel.

It seems that donut is seqing inside the map returned by the first component:

system/src/donut/system.cljc

Line 217 in a69fcae

(def ref-type (fn [v] (when (seqable? v) (first v))))

This triggers sp/chan->seq, which blocks.