Updated some comments.
Fixed some bugs.
Installed extra assertions.
Added `empty` method on range object.
Added generic front/back methods with a test.
Fixed some related bugs. Installed more assertions.
Updated to accept immutable pointer.
Disallow erase by immutable pointer again. I am not sure which one is right...
Made capatiy method to be constexpr.
Added constexpr capacity method.
Updated some comments.
Renamed `ObjectSlot` to `ListAtomSlot`.
The name ObjectSlot will be used in lower level class.
Added new ObjectSlot class. Added memory alignment control support.
ListAtomSlot also supports memory alignment, but ListAtomSlotIterator
and ListAtomSlotRange don't support alignment yet.