MapInto: relax Debug/Clone bounds

[Philippe-Cholet]

Derive those adds a bound on U which is not needed (I checked with the "expand macros" tool of the Rust playground) since it's only PhantomData.
Note that PhantomData<T> is Debug + Clone for any T.

I can hit easily hit the error (playground).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.55%. Comparing base (6814180) to head (dfa721f).
Report is 26 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #889      +/-   ##
==========================================
+ Coverage   94.38%   94.55%   +0.16%     
==========================================
  Files          48       48              
  Lines        6665     6753      +88     
==========================================
+ Hits         6291     6385      +94     
+ Misses        374      368       -6     

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