Parse bare XPath node tests as child axes.

When parsing a bare node test such as "A" this is now parsed as following:

    (axis "child" (test nil "A"))

Instead of this:

    (test nil "A")

According to the XPath specification both are identical and this simplifies some
of the code in the XPath evaluator.

lib/oga/xpath/parser.y

@@ -34,7 +34,7 @@ rule
     ;
 
   expression
-    : node_test
+    : node_test_as_axis
     | operator
     | axis
     | string
@@ -45,7 +45,7 @@ rule
     ;
 
   path_member
-    : node_test
+    : node_test_as_axis
     | axis
     | call
     ;
@@ -66,6 +66,13 @@ rule
     | T_SLASH path_member  { s(:absolute_path, val[1]) }
     ;
 
+  # Whenever a bare test is used (e.g. just "A") this actually means "child::A".
+  # Handling this on lexer level requires a look-behind/look-ahead while solving
+  # this on evaluator level produces inconsistent/confusing code.
+  node_test_as_axis
+    : node_test { s(:axis, 'child', val[0]) }
+    ;
+
   node_test
     : node_name           { s(:test, *val[0]) }
     | node_name predicate { s(:test, *val[0], val[1]) }