Add C++ concept for PAL

This will not be used unless the C++ standard version is raised to 20.  As
concepts and C++20 more generally are quite new, this does not do so.
Nevertheless, the use of concepts can improve the local development experience
as type mismatches are discovered earlier (at template invocation rather than
only during expansion).

src/mem/pool.h

@@ -2,6 +2,7 @@
 
 #include "../ds/flaglock.h"
 #include "../ds/mpmcstack.h"
+#include "../pal/pal_concept.h"
 #include "pooled.h"
 
 namespace snmalloc
@@ -21,7 +22,7 @@ namespace snmalloc
   {
   private:
     friend Pooled<T>;
-    template<typename TT>
+    template<SNMALLOC_CONCEPT(ConceptPAL) PAL>
     friend class MemoryProviderStateMixin;
 
     std::atomic_flag lock = ATOMIC_FLAG_INIT;