Add :root and :in selectors, fix variable bug

[mtdowling]

This commit adds support for the :in and :root functions. The :in function is a simpler way to check if a shape is in an expression, typically used to test if a variable contains a shape or if a root expression contains a shape. :root is used to create rooted common subexpressions that are evaluated once against every shape in the model. :root expressions are evaluate in an isolated context, so any variables used or stored by them are not accessible outside the root selector. :root selectors allows selection to be broken into multiple steps and evaluate globally.

Let's say you want all number shapes that are used in operation inputs, but not used in operation outputs. This can be done today using the following expression:

service
$outputs(~> operation -[output]-> ~> number)
~> operation -[input]-> ~> number
:not([@: @{id} = @{var|outputs|id}])

With the addition of the ``:in` selector, this gets easier because we can avoid using a scoped attribute selector:

service
$outputs(~> operation -[output]-> ~> number)
~> operation -[input]-> ~> number
:not(:in(${outputs}))

(Note: to make this work, I had to uncover and fix a bug in the implementation of how we store variables. We previously used Collection#add as a Receiver, but that method will return false if it's already seen a shape, which is wrong.)

With the addition of :root, you can use a much simpler expression:

number
:in(:root(service ~> operation -[input]-> ~> number))
:not(:in(:root(service ~> operation -[output]-> ~> number)))

(Note: the result of root expressions are run once and cached. No need to store them in a variable)

These expressions seem to be exactly the same, however, the :root expression gives a different result when working with models that contain multiple services. In the first two expressions, if any service uses shape X in input and not output, then X is a result. However, in the :root expression, X is only part of the result if no service uses it in their output shape closures.

Issue #, if available:

Description of changes:

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