[pointer][invariant] Remove AliasingMapping, Inaccessible

[joshlf]

We previously used AliasingMappings and Inaccessible to model UnsafeCell agreement. This abuses the notion of a mapping since one doesn't ever actually want to change the aliasing of a pointer (and certainly not to Inaccessible) - really this was meant to model pointer casts which should never be performed. In addition to being an awkward fit, the presence of Inaccessible meant that code could not assume that any Aliasing invariant permitted reading, and so we had to add extra machinery to work around this.

Future commits will use a different, simpler model for denoting UnsafeCell agreement or disagreement.

While we're here, make Read slightly more permissive, implemented for A: Aliasing, T: Immutable rather than just A: Reference, T: Immutable.

Makes progress on #1122, #1866

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This PR is on branch ptr-overhaul-more-generic-issue-1945.

• #1970
• #1969
• #1942
• #1911

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 89.46%. Comparing base (4bd33fb) to head (ac629d9).

Additional details and impacted files

@@                            Coverage Diff                             @@
##           Ifc49755af0d90eeefe7822d755d508403c266bda    #1942   +/-   ##
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  Coverage                                      89.46%   89.46%           
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  Files                                             17       17           
  Lines                                           5838     5838           
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  Hits                                            5223     5223           
  Misses                                           615      615           

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