Wrapped value viewer

[zampino]

Closes #639. Closes #638.

Fixes presentation of wrapped values, so far:

(viewer/present {:nextjournal/value (clerk/with-viewer viewer/string-viewer "hello")})

;; => [{:path [],
;;      :nextjournal/value "hello",
;;      :nextjournal/viewer {:name nextjournal.clerk.viewer/map-viewer,
;;                           :render-fn #viewer-fnnextjournal.clerk.render/render-map,
;;                           :opening-paren "{",
;;                           :closing-paren ("}"),
;;                           :page-size 10,
;;                           :hash "5drSiYgxwbGAGQx15hZm4Ts9vK16be"}}

the inner value and the assigned viewer were out of sync. This change makes viewer/present invariant of further wrapping.