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

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

src/mem/alloc.h

@@ -284,6 +284,7 @@ namespace snmalloc
       UNUSED(size);
       return free(p);
 #else
+      SNMALLOC_ASSERT(p != nullptr);
       check_size(p, size);
       if (likely((size - 1) <= (sizeclass_to_size(NUM_SMALL_CLASSES - 1) - 1)))
       {

src/mem/pagemap.h

@@ -188,7 +188,7 @@ namespace snmalloc
       {
         PagemapEntry* value = get_node<create_addr>(e, result);
         if (unlikely(!result))
-          return std::pair(nullptr, 0);
+          return {nullptr, 0};
 
         shift -= BITS_PER_INDEX_LEVEL;
         ix = (static_cast<size_t>(addr) >> shift) & ENTRIES_MASK;
@@ -208,11 +208,11 @@ namespace snmalloc
       Leaf* leaf = reinterpret_cast<Leaf*>(get_node<create_addr>(e, result));
 
       if (unlikely(!result))
-        return std::pair(nullptr, 0);
+        return {nullptr, 0};
 
       shift -= BITS_FOR_LEAF;
       ix = (static_cast<size_t>(addr) >> shift) & LEAF_MASK;
-      return std::pair(leaf, ix);
+      return {leaf, ix};
     }
 
     template<bool create_addr>

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;

src/mem/remoteallocator.h

@@ -19,7 +19,7 @@ namespace snmalloc
     union
     {
       Remote* non_atomic_next;
-      std::atomic<Remote*> next = nullptr;
+      std::atomic<Remote*> next{nullptr};
     };
 
     alloc_id_t allocator_id;

src/mem/slowalloc.h

@@ -63,6 +63,6 @@ namespace snmalloc
    */
   inline SlowAllocator get_slow_allocator()
   {
-    return SlowAllocator{};
+    return {};
   }
 } // namespace snmalloc