Rollup merge of #23025 - huonw:better-iter-infer, r=Gankro

 This concretely improves type inference of some cases (see included
test). I assume the compiler struggles to reason about multiple layers
of generic type parameters (even with associated-type equalities) but
*can* understand pure associated types, since they are always directly
computable from the input types.

Thanks to @shepmaster for noticing the issue with `Cloned` (I took that example as a test case).

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