Rework tree generation mode and API

Store tree nodes in congruent, compact arena array.
Define handle types to refer to tree nodes rather than pointers to
structure instances.
Free tree with context.

spec/test_token_user_fields.c

@@ -18,29 +18,22 @@ int main()
     p_context_t * context;
     context = p_context_new((uint8_t const *)input, strlen(input));
     assert(p_parse(context) == P_SUCCESS);
-    Start * start = p_result(context);
-    assert(start->pIDs);
-    assert(start->pIDs->id);
-#ifdef __cplusplus
-    assert(start->pIDs->id->comments == "# c1\n#  c2\n");
-#else
-    assert(start->pIDs->id->comments);
-    assert(strcmp(start->pIDs->id->comments, "# c1\n#  c2\n") == 0);
-#endif
-    assert(start->pIDs->pIDs);
-    assert(start->pIDs->pIDs->id);
-#ifdef __cplusplus
-    assert(start->pIDs->pIDs->id->comments == "# s1\n#   s2\n");
-#else
-    assert(start->pIDs->pIDs->id->comments);
-    assert(strcmp(start->pIDs->pIDs->id->comments, "# s1\n#   s2\n") == 0);
-#endif
+    Start start = p_result(context);
+    IDs ids = p_Start_pIDs(start);
+    assert(p_node_valid(ids));
+    Token id0 = p_IDs_id(ids);
+    assert(p_node_valid(id0));
+    assert(p_node_data(id0)->comments);
+    assert(strcmp(p_node_data(id0)->comments, "# c1\n#  c2\n") == 0);
+    IDs ids2 = p_IDs_pIDs(ids);
+    assert(p_node_valid(ids2));
+    Token id1 = p_IDs_id(ids2);
+    assert(p_node_valid(id1));
+    assert(p_node_data(id1)->comments);
+    assert(strcmp(p_node_data(id1)->comments, "# s1\n#   s2\n") == 0);
 
-#ifndef __cplusplus
     free(context->comments);
-#endif
     p_context_delete(context);
-    p_tree_delete(start);
 
     return 0;
 }