Configurable client meta-data (#662)

This provide a way to configure snmalloc to provide per object meta-data that is out of band. This can be used to provide different mitigations on top of snmalloc, such as storing memory tags in a compressed form, or provide a miracle pointer like feature.

This also includes a couple of TSAN fixes as it wasn't fully on in CI.

src/test/func/malloc/malloc.cc

@@ -375,6 +375,6 @@ int main(int argc, char** argv)
     our_malloc_usable_size(nullptr) == 0,
     "malloc_usable_size(nullptr) should be zero");
 
-  snmalloc::debug_check_empty<snmalloc::StandardConfig>();
+  snmalloc::debug_check_empty<snmalloc::Alloc::Config>();
   return 0;
 }