output-template: allow splicing statements instead of expression

if the text inside the delimiters begins with ; (a semicolon) then
the rest of it is expected to be one or more Lua statements. It needs
to say `return "foo"` to interpolate anything, as there is no
implicit return of the value of the last statement

pkgs/output-template/output-template.fnl

@@ -35,8 +35,11 @@
                }]
     (string.gsub text delim
                  (fn [x]
-                   (assert ((load (.. "return " x) x :t myenv))
+                   (let [chunk (if (= (x:sub 1 1) ";")
-                           (string.format "missing value for %q" x))))))
+                                   (x:sub 2)
+                                   (.. "return " x))]
+                     (assert ((load chunk x :t myenv))
+                             (string.format "missing value for %q" x)))))))
 
 (fn run []
   (let [[opening closing] arg
@@ -49,6 +52,11 @@
   (expect= (pick-values 1 (substitute "{{ json_quote(\"o'reilly\") }}" "{{" "}}"))
            "\"o'reilly\"")
 
+  (expect= (pick-values 1 (substitute "{{; local a=9; return a }}" "{{" "}}")) "9")
+
+  ;; "globals" set in one interpolation are available in subsequent ones
+  (expect= (pick-values 1 (substitute "{{; a=42; return a }} {{ a and 999 or 0 }}" "{{" "}}")) "42 999")
+
   (fn slurp [name]
     (with-open [f (assert (io.open name))] (f:read "*a")))
   (expect=