[pointer] Document validity safety invariant

[joshlf]

Document the semantics of Ptr validity invariants on the Validity
trait.

Makes progress on #2226, #1866

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This PR is on branch ptr-validity.

• [pointer] Support generic TransmuteFrom framework #2408
• [pointer] Document validity safety invariant #2403

[joshlf]

This draft lays the groundwork for implementing the semantics described in the commit message of #2397. As of this writing, all I've added is a safety comment on the Validity trait. This is clearly insufficient on its own - in order to be completely sound, there must be places where this invariant is consumed. (It needs to be proven to be upheld as well, but that's more obvious.) My hunch is that this will need to be at places where we actually do something with the raw pointer inside a Ptr (read from it, convert it to a vanilla reference, etc).

Update: On further reflection, I think we will need the following:

1. We document that validity provides a guarantee regarding the set of values that may appear in a Ptr's referent (already done)
2. We consume this guarantee wherever a Ptr's raw pointer is dereferenced or converted into a vanilla reference
3. We document that unsafe code is responsible for upholding this guarantee on any Ptr, including Ptrs that it produces (already done)
4. We document specific necessary conditions that must be satisfied in order for this guarantee to be upheld (specifically, the conditions under which the validity set may not be expanded or shrunk). This technically does not add any new information (it's implicit in the original guarantee from (1)), but it's useful to write it down so we don't forget it. As a result of being implicit, it's not explicitly consumed or discharged anywhere - but again, I think it's important for future readers since it's they are subtle requirements which are easy to miss.
5. We discharge the general form of the safety obligation (from (1)) wherever unsafe code creates Ptrs or performs operations which affect what values can be written to the referents of existing Ptrs. This discharge will likely entail repeating the conditions mentioned in (4).

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.33%. Comparing base (5aa0bca) to head (9a5a4dd).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2403      +/-   ##
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+ Coverage   87.21%   87.33%   +0.12%     
==========================================
  Files          17       17              
  Lines        6390     6451      +61     
==========================================
+ Hits         5573     5634      +61     
  Misses        817      817              

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