Rollup merge of #53393 - BurntPizza:serialize-inlines, r=alexcrichton

Mark libserialize functions as inline

Got to thinking: "what if that big pile of tiny functions isn't inlining as it should?"
So a few `replace-regex` later the local perf run says this:
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![](https://i.imgur.com/gvdJEgG.png)
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Not huge, but still a win, which is interesting. Want to verify with the real perf run, but I understand there's a backlog.

I didn't notice any increase in compile time or binary sizes for rustc/libs.

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