Add p_context_delete() and p_tree_delete() for D targets

spec/test_tree_invalid_positions.d

@@ -10,8 +10,7 @@ int main()
 unittest
 {
     string input = "\na\n  bb ccc";
-    p_context_t * context;
-    context = p_context_new(input);
+    p_context_t * context = p_context_new(input);
     assert(p_parse(context) == P_SUCCESS);
     Start * start = p_result(context);
 
@@ -34,6 +33,8 @@ unittest
     assert_eq(3, start.end_position.row);
     assert_eq(8, start.end_position.col);
 
+    p_tree_delete(start);
+
     input = "a\nbb";
     context = p_context_new(input);
     assert(p_parse(context) == P_SUCCESS);
@@ -58,6 +59,8 @@ unittest
     assert_eq(2, start.end_position.row);
     assert_eq(2, start.end_position.col);
 
+    p_tree_delete(start);
+
     input = "a\nc\nc";
     context = p_context_new(input);
     assert(p_parse(context) == P_SUCCESS);
@@ -82,6 +85,8 @@ unittest
     assert_eq(3, start.end_position.row);
     assert_eq(1, start.end_position.col);
 
+    p_tree_delete(start);
+
     input = "a";
     context = p_context_new(input);
     assert(p_parse(context) == P_SUCCESS);
@@ -101,4 +106,6 @@ unittest
     assert_eq(1, start.position.col);
     assert_eq(1, start.end_position.row);
     assert_eq(1, start.end_position.col);
+
+    p_tree_delete(start);
 }