rust-discovery: encapsulated conversions to vec for instance methods
This new `pyiter_to_vec` is pretty trivial, and only mildly reduces
code duplication. The main advantage is that it encapsulates access
to the `index` attribute, which will be changed when we replace the
C index by the Rust index, given as `PySharedRef`.
# So GitLab doesn't think we're using tons of Perl
*.t -linguist-detectable