Fix/ignore warnings within doctests

[Philippe-Cholet]

#963 lead me to know about #![doc(test(attr(...)))].

Our doctests have some warnings that are ignored by default.
Here, I fix some, sometimes silently (comment out), and globally ignore deprecated and unstable_name_collisions.
Maybe I should allow(unused_variables) instead of many # let _ = ...;?

[codecov]

Codecov Report

Attention: Patch coverage is 91.30435% with 2 lines in your changes missing coverage. Please review.

Project coverage is 94.51%. Comparing base (6814180) to head (9238e2a).
Report is 108 commits behind head on master.

Files	Patch %	Lines
src/lib.rs	87.50%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #964      +/-   ##
==========================================
+ Coverage   94.38%   94.51%   +0.12%     
==========================================
  Files          48       49       +1     
  Lines        6665     7072     +407     
==========================================
+ Hits         6291     6684     +393     
- Misses        374      388      +14     

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