Fix bug 833666, "scour does not clean comments if file starts with a comment", by correctly and efficiently iterating over the elements of a live sequence. Unit tests are updated.

unittests/comment-beside-xml-decl.svg

@@ -1,4 +1,10 @@
 <?xml version="1.0" encoding="utf-8" standalone="yes"?>
 <!-- Oh look a comment -->
+<!-- generated by foobar version 20120503 -->
+<!-- And another -->
 <svg xmlns="http://www.w3.org/2000/svg">
+  <!-- This comment is meant to test whether removing a comment before <svg>
+       messes up removing comments thereafter -->
+  <!-- And this one is meant to test whether iteration works correctly in
+       <svg> as well as the document element -->
 </svg>