fix: Link and neighbour iterators counting self-loops twice

[aborgna-q]

The all_links iterator for both Portgraph and Multiportgraph, and all_neighbours for the latter yielded self-loops twice.

This in turn caused insert_graph to try inserting repeated links, as reported in #130.

This PR fixes the error and adds tests for that.

Fixes #130.

drive-by: Add missing checks in the pre-commit hook.

[codecov]

Codecov Report

Attention: Patch coverage is 94.63415% with 11 lines in your changes are missing coverage. Please review.

Project coverage is 80.17%. Comparing base (99a2bfc) to head (f96c5b5).

Files	Patch %	Lines
src/portgraph.rs	88.23%	3 Missing and 3 partials ⚠️
src/multiportgraph.rs	94.50%	0 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #132      +/-   ##
==========================================
+ Coverage   79.38%   80.17%   +0.79%     
==========================================
  Files          21       21              
  Lines        5016     5176     +160     
  Branches     5016     5176     +160     
==========================================
+ Hits         3982     4150     +168     
+ Misses        962      949      -13     
- Partials       72       77       +5     

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