A collection of blocks

A number of issues recently have started to imply that we need an abstraction that represents a collection of blocks. There are use cases where it's natural to want the visibility of an identifier to be shared within a collection of blocks but not outside it. On templates, it has a nice feel to be able to use the name of that collection when referencing a state on a block or class within it, instead of the `state` namespace.

The app would be one such default collection (though it may be good for complex apps to divide it into several major groups). blocks provided by a node module would also form a natural boundary around a group of blocks.

We need to write an RFC for this capability.

Start with the issue's use cases for shared visibility, especially the app and node-module boundaries, and review the repository context needed to describe a collection-of-blocks abstraction. The work is done when an RFC clearly defines the capability, its scope, and how these example collections would behave.

Written by the indexing model from the issue text.