Various minor changes to aid compiling with std14 (#182)

These changes make the code compile in clang10 with -std14.

src/mem/pooled.h

@@ -14,7 +14,7 @@ namespace snmalloc
     friend class MPMCStack;
 
     /// Used by the pool for chaining together entries when not in use.
-    std::atomic<T*> next = nullptr;
+    std::atomic<T*> next{nullptr};
     /// Used by the pool to keep the list of all entries ever created.
     T* list_next;
     std::atomic_flag in_use = ATOMIC_FLAG_INIT;