Improve precision when typechecking `in`

[john-h-kastner-aws]

Category

Cedar validation features

Describe the feature you'd like to request

The validator sometimes gives imprecise types to expressions using in.

• principal in [] will type as Bool when principal is Unspecified, although we could give the more precise type of False.
• Expressions like action in action or principal in principal will type as Bool, even though we could give them type True.
• If the action hierarchy says that Action::"read" is a descendant of Action::"read_or_write", then Action::"read" in Action::"read_or_write" will type as Bool, even though we could type it as True.

Describe the solution you'd like

n/a

Describe alternatives you've considered

n/a

Additional context

No response

Is this something that you'd be interested in working on?

• 👋 I may be able to implement this feature request
• ⚠️ This feature might incur a breaking change

[john-h-kastner-aws]

From #310, a substantial improvement can be had by implementing singleton action types. This would make giving precise types to in expression involving actions trivial, even simplifying some of the existing code. This will also support precise types for in expression in policy bodies that aren't directly applied to action literals.