multidict C Extension Analysis Report

I ran a [code analysis tool](https://github.com/devdanzin/cext-review-toolkit) I'm developing on multidict and it generated an interesting report: https://gist.github.com/devdanzin/a36588ae7e2b73f0d85f8a925fb13b3d. It contains issue descriptions, suggested fixes, and reproducers for some of the issues.

From the summary:

> The most critical finding is a **double-free/use-after-free in `md_contains`** and a **crash from swapped format string arguments** in `_err_cannot_fetch`. The iterator and view types are also missing the heap type lifecycle management (`Py_VISIT(Py_TYPE(self))`) that the main dict types correctly implement.

I hope it helps.