Update Internal Stack Behavior

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'
```

Start by locating the implementations of allow_phi!, disallow_phi!, allow_phi, and disallow_phi and read how the current raw stack is maintained. Trace the block behavior and verify that GUIDs are returned, optionally accepted for removal, and tied to the matching block access so the mixed example leaves the expected access active.

Written by the indexing model from the issue text.