Lua filter: use destructured functions for inline filters

Instead of taking the whole inline element, forcing users to destructure it
themselves, the components of the elements are passed to the filtering
functions.

test/lua/strmacro.lua

@@ -1,10 +1,11 @@
 return {
-  { Str = function (inline)
-      if inline.c == "{{helloworld}}" then
+  {
+    Str = function (str)
+      if str == "{{helloworld}}" then
         return pandoc.Emph {pandoc.Str "Hello, World"}
       else
-        return inline
+        return pandoc.Str(str)
       end
-  end,
+    end,
   }
 }