stage2 大量优化代码使其自举编译时间大幅缩短

stage2/def.spl

@@ -87,11 +87,7 @@ fn drop(self: *DefTable) {
 // id 0 与越界均返回 null（对应 stage1 spl_def_node）。
 fn node(self: *DefTable, id: usize): *DefNode {
     if id == 0 || id >= self.table.len { ret null; }
-    match Vec.get(&self.table, id) {
-        .Ok(ptr) => { ret ptr as *DefNode; },
-        .Err(error) => { ret null; },
-    }
-    ret null;
+    ret (self.table.data as *DefNode) + id;
 }
 
 fn count(self: *DefTable): usize {