Address review comments.

Move slow_allocator into a separate header in the snmalloc namespace and
rename it for consistency with the rest of the codebase.  Delete its
copy and move constructors / assignment operators.

src/pal/pal_freebsd.h

@@ -4,10 +4,10 @@
 #  include "../ds/bits.h"
 #  include "../mem/allocconfig.h"
 
-#  include <sys/mman.h>
-#  include <strings.h>
-#  include <stdio.h>
 #  include <pthread.h>
+#  include <stdio.h>
+#  include <strings.h>
+#  include <sys/mman.h>
 
 namespace snmalloc
 {