Provide a generic PAL feature detection mechanism.

We can support up to 64 optional features, currently only one is used.

src/mem/alloc.h

@@ -1004,7 +1004,7 @@ namespace snmalloc
             static_assert(
               std::remove_reference_t<decltype(
                 large_allocator
-                  .memory_provider)>::supports_low_memory_notification,
+                  .memory_provider)>::pal_supports<LowMemoryNotification>(),
               "A lazy decommit strategy cannot be implemented on platforms "
               "without low memory notifications");
           }

src/mem/largealloc.h

@@ -47,7 +47,6 @@ namespace snmalloc
   template<class PAL>
   class MemoryProviderStateMixin : public PAL
   {
-    using PAL::supports_low_memory_notification;
     std::atomic_flag lock = ATOMIC_FLAG_INIT;
     size_t bump;
     size_t remaining;
@@ -117,13 +116,21 @@ namespace snmalloc
       lazy_decommit_guard.clear();
     }
 
+  public:
+    template<PalFeatures F, typename P = PAL>
+    constexpr static bool pal_supports()
+    {
+      return (P::pal_features & F) == F;
+    }
+
+  private:
     /**
      * Wrapper that is instantiated only if the memory provider supports low
      * memory notifications and forwards the call to the memory provider.
      */
     template<typename M>
     ALWAYSINLINE uint64_t low_mem_epoch(
-      std::enable_if_t<M::supports_low_memory_notification, int> = 0)
+      std::enable_if_t<pal_supports<LowMemoryNotification, M>(), int> = 0)
     {
       return PAL::low_memory_epoch();
     }
@@ -135,7 +142,7 @@ namespace snmalloc
      */
     template<typename M>
     ALWAYSINLINE uint64_t low_memory_epoch(
-      std::enable_if_t<!M::supports_low_memory_notification, int> = 0)
+      std::enable_if_t<!pal_supports<LowMemoryNotification, M>(), int> = 0)
     {
       return 0;
     }