update internal terminology: `Substs` -> `GenericArgs`

[lcnr]

change substitutions to the more familiar name of generic arguments in the compiler.

this work was started in #113591 but there are still some uses of the old terminology left.

How to work on this

grep for remaining uses of subst, substitution and substitute and figure out how to replace them with the new terminology: instantiate and generic argument

cc #110254 @oli-obk

[lcnr]

This does require some familiarity with the compiler as I assume it to be very annoying to rebase in case there are issues with the implementation.

@lcnr can i work on this?

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[mdibaiee]

@AnakinSkywalkeer are you still working on this? If not I would like to work on this.

@AnakinSkywalkeer are you still working on this? If not I would like to work on this.

sure

[mdibaiee]

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