Refactor json_pack()

* Implement a "scanner" that reads the format string, maintaining state

* Split json_vnpack() to three separate functions for packing objects,
  arrays and simple values. This makes it more clear what is being
  packed, and the object and array structures become more evident.

* Make the skipping of ignored character simpler, i.e. skip ':' and
  ',' independent of their context

This patch shaves around 80 lines of code from the original
implementation.

test/suites/api/test_pack.c

@@ -191,7 +191,7 @@ int main()
 
     if(json_pack(&error, "{ s: {},  s:[ii{} }", "foo", "bar", 12, 13))
         fail("json_pack failed to catch missing ]");
-    if(error.line != 1 || error.column != 13)
+    if(error.line != 1 || error.column != 19)
         fail("json_pack didn't get the error coordinates right!");
 
     if(json_pack(&error, "[[[[[   [[[[[  [[[[ }]]]] ]]]] ]]]]]"))