hoon: memoize rend:co

We commonly print many names in a row, often the same ones.  For
example, on landscape's initial load, we send all the members of all the
groups we're in, and there's substantial overlap.

At least half the cost is in +fein, which is not currently jetted, but I
believe there's an old jet in the git history.

pkg/arvo/sys/hoon.hoon

@@ -5455,9 +5455,10 @@
   ~%  %co  ..co  ~
   =<  |_  lot=coin
       ++  rear  |=(rom=tape rend(rep rom))
-      ++  rent  `@ta`(rap 3 rend)
+      ++  rent  ~+  `@ta`(rap 3 rend)
       ++  rend
         ^-  tape
+        ~+
         ?:  ?=(%blob -.lot)
           ['~' '0' ((v-co 1) (jam p.lot))]
         ?:  ?=(%many -.lot)