Cleanup: Consistently use `Param` instead of `Arg`

[Centril]

For example, ideally fn visit_arg would be named fn visit_param since this is about formal parameters (in function signatures and from the POV inside the function body) instead of actual arguments (the values you pass to functions).

The type Arg should probably be named Param.

(This is not an exhaustive list, there are other places "arg" is used not mentioned here that should probably use "param" instead)

cc @petrochenkov @c410-f3r
cc #61856

This issue has been assigned to @kper via this comment.

[kper]

Can I work on this?

[rustbot]

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[csmoe]

@kper you can assign to yourself with @rustbot claim.

[kper]

@rustbot kper

[csmoe]

@kper it's already "assign"ed to you.