Make ptr_eq public

[arthurprs]

As discussed previously in #80. This should be a valid public api in persistent data-structures. It's already in im and in the stdlib.

It's valuable in a some cases as an identity check but also as a fast-path to avoid expensive set computations (intersection, union, etc.). In our application, we take great advantage of the latter.

Furthermore, persistent data structures in practice always(?) have efficient ptr_eq implementations, otherwise it would imply cloning isn't cheap or can't be done with a non-mut pointer. So it's likely to be future-proof even.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (dc024b5) 96.29% compared to head (6ac8cbd) 96.29%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #91   +/-   ##
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  Coverage   96.29%   96.29%           
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  Files          11       11           
  Lines        1645     1645           
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  Hits         1584     1584           
  Misses         61       61           

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