Auto merge of #15224 - HKalbasi:mir, r=HKalbasi

Replace `x` with `it`

I kept some usages of `x`:
* `x`s that are used together with `y`, `z`, ...
* `x` that shadow `it`. I use `it` for iterators out of r-a, so there were some cases that I used `it` and `x` together.
* `x` in test fixtures. Many of those `x` usages was not me so I thought it's better to keep them as is.

I tried to remove the rest, but since there was too many `x` I might missed some of them or changed some of them that I didn't want to change.

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