add wrapper methods to reserve more stacks to existing schedulers

ostd/concurrency.hh

@@ -345,6 +345,10 @@ public:
         p_stacks.deallocate(st);
     }
 
+    void reserve_stacks(size_t n) {
+        p_stacks.reserve(n);
+    }
+
 private:
     void dispatch() {
         while (!p_coros.empty()) {
@@ -508,6 +512,10 @@ public:
         p_stacks.deallocate(st);
     }
 
+    void reserve_stacks(size_t n) {
+        p_stacks.reserve(n);
+    }
+
 private:
     template<typename TSA, typename F, typename ...A>
     void spawn_add(TSA &&sa, F &&func, A &&...args) {