Overhaul CapPtr

* Switch to a multidimensional taxonomy.

  Rather than encoding the abstract bound states in a single enum, move to a
  more algebraic treatment.  The dimensions themselves are within the
  snmalloc::capptr_bounds namespace so that their fairly generic names do not
  conflict with consumer code.  Aliases for many points in the space are
  established outside that namespace for ease of use elsewhere.

* Introduce several new namespaces:

    * snmalloc::capptr::dimension holds each of the dimension enums

    * snmalloc::capptr holds the bound<> type itself and a ConceptBound

    * snmalloc::capptr::bounds gives convenient specializations of bound<>

    * snmalloc::capptr also has aliases for CapPtr<> itself

  All told, rather than `CapPtr<T, CBChunk>`, we now expect client code to read
  `capptr::Chunk<T>` in almost all cases (and this is just an alias for the
  appropriate `CapPtr<T, bounds<...>>` type).  When the bound<>s themselves are
  necessary, as when calling capptr_bound, we expect that they will almost
  always be pronounced using an alias (e.g., `capptr::bounds::Alloc`).

* Chase consequences.

* Prune old taxa and aliases that are no longer in use in snmalloc2.

src/mem/localalloc.h

@@ -263,7 +263,9 @@ namespace snmalloc
         MetaEntry entry = SharedStateHandle::Pagemap::get_metaentry(
           core_alloc->backend_state_ptr(), address_cast(p));
         local_cache.remote_dealloc_cache.template dealloc<sizeof(CoreAlloc)>(
-          entry.get_remote()->trunc_id(), CapPtr<void, CBAlloc>(p), key_global);
+          entry.get_remote()->trunc_id(),
+          capptr::AllocFull<void>(p),
+          key_global);
         post_remote_cache();
         return;
       }
@@ -469,7 +471,7 @@ namespace snmalloc
         {
           local_cache.remote_dealloc_cache.template dealloc<sizeof(CoreAlloc)>(
             entry.get_remote()->trunc_id(),
-            CapPtr<void, CBAlloc>(p),
+            capptr::AllocFull<void>(p),
             key_global);
 #  ifdef SNMALLOC_TRACING
           std::cout << "Remote dealloc fast" << p << " size " << alloc_size(p)
@@ -501,7 +503,7 @@ namespace snmalloc
 #  endif
         ChunkRecord* slab_record =
           reinterpret_cast<ChunkRecord*>(entry.get_metaslab());
-        slab_record->chunk = CapPtr<void, CBChunk>(p);
+        slab_record->chunk = capptr::Chunk<void>(p);
         check_init(
           [](
             CoreAlloc* core_alloc,