localalloc::dealloc tweak size math for large objects

Avoid computing bits::next_pow2_bits(1 << n).  Even if the compiler can see
through enough of the algebra, it's surely more direct to just use n.

While here, slightly expand documentation about what's going on with the
"sizeclass" encoded into MetaEntry-s.

src/backend/address_space_core.h

@@ -77,13 +77,16 @@ namespace snmalloc
     {
       if (align_bits >= MIN_CHUNK_BITS)
       {
-        // The pagemap stores MetaEntrys, abuse the metaslab field to be the
+        // The pagemap stores `MetaEntry`s; abuse the metaslab field to be the
         // next block in the stack of blocks.
         //
         // The pagemap entries here have nullptr (i.e., fake_large_remote) as
         // their remote, and so other accesses to the pagemap (by
         // external_pointer, for example) will not attempt to follow this
         // "Metaslab" pointer.
+        //
+        // dealloc() can reject attempts to free such MetaEntry-s due to the
+        // zero sizeclass.
         MetaEntry t(reinterpret_cast<Metaslab*>(next.unsafe_ptr()), nullptr, 0);
         Pagemap::set_metaentry(local_state, address_cast(base), 1, t);
         return;