Rename node accessor methods

Instead of child() vs concrete_child(), next_sibling() vs next_concrete_sibling(), etc,
the default is switched: child() refers to the concrete syntax tree, and named_child()
refers to the AST. Because the AST is abstract through exclusion of some nodes, the
names are clearer if the qualifier goes on the AST operations

spec/runtime/stack_spec.cc

@@ -32,7 +32,7 @@ describe("stacks", [&]() {
           sym1,
           ts_length_make(0, 0),
           ts_length_make(1, 1),
-          TSNodeTypeNormal);
+          TSNodeTypeNamed);
 
       ts_stack_push(&stack, 5, node1);
     });