Add a method to collection views to return the number of items

[jvff]

Motivation

In some views, the only way to discover the amount of items in them is to use for_each_key and count them. This pattern is common enough that it could be a method in the views themselves.

Proposal

Add a helper method to the views that store multiple items, and name it

• len if the number of items is obtained O(1) (e.g. it is persisted in storage)
• count if it needs to be calculated in O(n) (e.g., with the for_each_key described above)