Update Internal Stack Behavior

[egreer]

Instead of using a raw stack with simple push and pops we should assign each entry a GUID so that we can have more controlled revoke behavior for easier mixing of block and ! syntax's.

• allow_phi! should return a GUID
• disallow_phi! should accept an optional GUID to remove
• allow_phi and disallow_phi blocks should track the GUID they create and then revoke that particular access.

This will better support weird mixes like the following, with at least consistent behavior (even if we still don't recommend it):

patient_john = PatientInfo.new

guid = patient_john.allow_phi!('allow1', 'reason)      # Stack: 'allow1'

patient_john.disallow_phi do     # Stack: 'allow1', 'disallow1'
   patient_john.disallow_phi(guid)    # Stack: 'disallow1'
   guid = patient_john.allow_phi('allow2')    # Stack: 'disallow1',  'allow2'
end

patient_john.name # Stack: 'allow2'