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_start_rule_tree.c

@@ -9,11 +9,10 @@ int main()
     p_context_t * context;
     context = p_context_new((uint8_t const *)input, strlen(input));
     assert_eq(P_SUCCESS, p_parse(context));
-    Top * top = p_result(context);
-    assert(top->pToken != NULL);
-    assert_eq(TOKEN_hi, top->pToken->token);
+    Top top = p_result(context);
+    assert(p_node_valid(p_Top_pToken(top)));
+    assert_eq(TOKEN_hi, p_tree_walk_Top(top, pToken, token));
 
-    p_tree_delete(top);
     p_context_delete(context);
 
     return 0;