expose Vector->reserve(size) to php space

example usage: genv() takes a Vector of Awaitable and creates a
Vector of WaitHandles. resize() requires a default value, reserve allows us to hint proper allocation.

hphp/runtime/ext/ext_collections.cpp

@@ -256,6 +256,21 @@ void c_Vector::t_resize(CVarRef sz, CVarRef value) {
   resize(intSz, val);
 }
 
+void c_Vector::t_reserve(CVarRef sz) {
+  if (!sz.isInteger()) {
+    Object e(SystemLib::AllocInvalidArgumentExceptionObject(
+      "Parameter sz must be a non-negative integer"));
+    throw e;
+  }
+  int64_t intSz = sz.toInt64();
+  if (intSz < 0) {
+    Object e(SystemLib::AllocInvalidArgumentExceptionObject(
+      "Parameter sz must be a non-negative integer"));
+    throw e;
+  }
+  reserve(intSz);
+}
+
 Object c_Vector::t_clear() {
   ++m_version;
   uint sz = m_size;

hphp/runtime/ext/ext_collections.h

@@ -46,6 +46,7 @@ class c_Vector : public ExtObjectDataFlags<ObjectData::VectorAttrInit|
   Object t_append(CVarRef val); // deprecated
   Variant t_pop();
   void t_resize(CVarRef sz, CVarRef value);
+  void t_reserve(CVarRef sz);
   Object t_clear();
   bool t_isempty();
   int64_t t_count();

hphp/system/idl/collections.idl.json

@@ -331,6 +331,21 @@
                         }
                     ]
                 },
+                {
+                    "name": "reserve",
+                    "flags": [
+                        "HasDocComment"
+                    ],
+                    "return": {
+                        "type": null
+                    },
+                    "args": [
+                        {
+                            "name": "sz",
+                            "type": "Variant"
+                        }
+                    ]
+                },
                 {
                     "name": "toArray",
                     "flags": [