fix(util): add missing file

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>

src/util/fixed_size_allocator.h

@@ -0,0 +1,39 @@
+/*
+Copyright (c) 2014 Microsoft Corporation. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+
+Author: Leonardo de Moura
+*/
+#pragma once
+#include "util/memory.h"
+
+namespace lean {
+template<unsigned Size>
+class fixed_size_allocator {
+    void * m_free_list;
+public:
+    fixed_size_allocator():m_free_list(nullptr) {}
+    ~fixed_size_allocator() {
+        while (m_free_list != nullptr) {
+            void * r = m_free_list;
+            m_free_list = *(reinterpret_cast<void **>(r));
+            free(r);
+        }
+    }
+
+    void * allocate() {
+        if (m_free_list != nullptr) {
+            void * r = m_free_list;
+            m_free_list = *(reinterpret_cast<void **>(r));
+            return r;
+        } else {
+            return malloc(Size);
+        }
+    }
+
+    void recyle(void * ptr) {
+        *(reinterpret_cast<void**>(ptr)) = m_free_list;
+        m_free_list = ptr;
+    }
+};
+}