Fix the remaining clang-tidy warnings.

This introduces a new `address_t` type and two new casts: `pointer_cast`
and `address_cast` for casting between an `address_t` and a pointer.
These should make it easier to audit the codebase for casts between
pointers and integers.  In particular, the remaining `reinterpret_cast`s
and `pointer_cast`s should be the only places where we could perform
invalid pointer arithmetic.

Also adds a `pointer_offset` helper that adds an offset (in bytes) to a
pointer, preserving its original type.  This is a sufficiently common
pattern that it seemed worthwhile to centralise it.

src/mem/allocstats.h

@@ -82,7 +82,8 @@ namespace snmalloc
 
         if (slab_count.current != 0)
         {
-          double occupancy = (double)count.current / (double)slab_count.current;
+          double occupancy = static_cast<double>(count.current) /
+            static_cast<double>(slab_count.current);
           uint64_t duration = now - time;
 
           if (ticks == 0)
@@ -103,7 +104,7 @@ namespace snmalloc
         // Keep in sync with header lower down
         count.print(csv, multiplier);
         slab_count.print(csv, slab_multiplier);
-        size_t average = (size_t)(online_average * multiplier);
+        size_t average = static_cast<size_t>(online_average * multiplier);
 
         csv << average << (slab_multiplier - average) * slab_count.max
             << csv.endl;