[pointer] Make invariants opaque, more ergonomic

[joshlf]

Closes #1876

[joshlf]

@jswrenn I didn't update any methods which modify two of three invariants. Any galaxy brain ideas on how to make those ergonomic?

The best I could come up with was yet another set of associated types e.g.:

type RetainAliasing<A: Alignment, V: Validity> = ...;

...but that's not forwards-compatible with adding new invariant types.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.64%. Comparing base (51475bd) to head (029d5d6).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1895      +/-   ##
==========================================
- Coverage   87.65%   87.64%   -0.02%     
==========================================
  Files          17       17              
  Lines        5964     5956       -8     
==========================================
- Hits         5228     5220       -8     
  Misses        736      736              

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