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_named_optional_rule_component_tree.d

@@ -12,40 +12,40 @@ unittest
     string input = "b";
     p_context_t * context = p_context_new(input);
     assert(p_parse(context) == P_SUCCESS);
-    Start * start = p_result(context);
-    assert(start.pToken1 is null);
-    assert(start.pToken2 !is null);
+    Start start = p_result(context);
+    assert(!start.pToken1.valid);
+    assert(start.pToken2.valid);
     assert_eq(TOKEN_b, start.pToken2.token);
-    assert(start.pR3 is null);
-    assert(start.pR is null);
-    assert(start.r is null);
+    assert(!start.pR3.valid);
+    assert(!start.pR.valid);
+    assert(!start.r.valid);
 
-    p_tree_delete(start);
+    p_context_delete(context);
 
     input = "abcd";
     context = p_context_new(input);
     assert(p_parse(context) == P_SUCCESS);
     start = p_result(context);
-    assert(start.pToken1 != null);
+    assert(start.pToken1.valid);
     assert_eq(TOKEN_a, start.pToken1.token);
-    assert(start.pToken2 != null);
-    assert(start.pR3 != null);
-    assert(start.pR != null);
-    assert(start.r != null);
+    assert(start.pToken2.valid);
+    assert(start.pR3.valid);
+    assert(start.pR.valid);
+    assert(start.r.valid);
     assert(start.pR == start.pR3);
     assert(start.pR == start.r);
     assert_eq(TOKEN_c, start.pR.pToken1.token);
 
-    p_tree_delete(start);
+    p_context_delete(context);
 
     input = "bdc";
     context = p_context_new(input);
     assert(p_parse(context) == P_SUCCESS);
     start = p_result(context);
-    assert(start.pToken1 is null);
-    assert(start.pToken2 !is null);
-    assert(start.pR !is null);
+    assert(!start.pToken1.valid);
+    assert(start.pToken2.valid);
+    assert(start.pR.valid);
     assert_eq(TOKEN_d, start.pR.pToken1.token);
 
-    p_tree_delete(start);
+    p_context_delete(context);
 }