Add requirement-is-satisfied-by-id expression function

[hawkrives]

If a requirement has an "id" field, store it in a new structure on RequirementContext, requirement-state. Then allow the "requirement-is-satisfied-by-id" expression to check ctx.requirement-state, raising an error if the ID doesn't exist yet.

Requirements would need to populate this field when they are audited, I think.

Need to ensure uniqueness on the ID field.

Future optimization: check the full document for any expressions that need this value, and skip setting it if there are none.