Implement generic `LenType` in other containers.

[GnomedDev]

After #504 Vec now has a generic LenType which significantly improves space efficiency, but the other containers depending on Vec do not as to avoid the original PR becoming super wide-reaching and impossible to merge.

This is a tracking issue for adding LenType generics to the other containers.

• StringInner: (Add generic LenType to String #552)
• LinearMapInner
• BinaryHeapInner
• IndexMap/Set

The following containers use a usize length, but do not depend on Vec:

• DequeInner
• HistoryBufInner
• QueueInner

The following containers have their own implementation of LenType, and should probably be swapped over:

• SortedLinkedListInner (Migrate SortedLinkedList to LenType #553)