Pull solve_item_traits out of mir_traits

[W95Psp]

#627 introduces a use of mir_traits::solve_item_traits in the extraction of types, which are common to THIR and MIR. Thus solve_item_traits should move out of mir_traits.rs.

solve_item_traits should anyway be re-written using the traits rustc_utils::SubstBinder and rustc_utils::PredicateToPolyTraitRef.

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[W95Psp]

It's still to be done (https://github.com/hacspec/hax/blob/16db1cc4571cbe1d4818945224f2cd693bb1b32c/frontend/exporter/src/types/mir_traits.rs#L47)

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This issue has been marked as stale due to a lack of activity for 60 days. If you believe this issue is still relevant, please provide an update or comment to keep it open. Otherwise, it will be closed in 7 days.

[W95Psp]

@Nadrieril fixed that in #1031 :)