Janet subprocess execute: `exec-slurp` by the great @bakpakin

exec-slurp.janet

# See also andrewchambers/janet-sh for a more complete (and awesome) solution:
# https://acha.ninja/blog/dsl_for_shell_scripting/
#
# Source: https://github.com/janet-lang/jpm/blob/07b0c18daf75cfdf098eff5526a17ad0436e3dc0/jpm/shutil.janet#L123-L137
(defn exec-slurp
  "Read stdout of subprocess and return it trimmed in a string."
  [& args]
  (when (dyn :verbose)
    (flush)
    (print ;(interpose " " args)))
  (def env (patch-env))
  (put env :out :pipe)
  (def proc (os/spawn args :epx env))
  (def out (get proc :out))
  (def buf @"")
  (ev/gather
    (:read out :all buf)
    (:wait proc))
  (string/trimr buf))