Add dynamic-extent's for continuations?

[kchanqvq]

Currently screamer still CONSes a lot for continuation closures. On some implementations (e.g. SBCL), dynamic-extent declarations will prevent this. I wish I were more familiar with the internals of screamer -- my question is, is it the case that continuations in screamer never escape? i.e., I see screamer forms macroexpand to things like

(LET ((#:CONTINUATION-614
        #'(LAMBDA (&REST #:DUMMY-607)
            ...)))
  ...)

Is it always true that such #:CONTINUATION-614 is never used outside the dynamic extent of the LET form? If so, I will work on adding appropriate dynamic-extent declarations.