Add method `Impl::all_in_module(…)` for allowing more localized querying
This PR is motivated by an [outside use](https://github.com/regexident/cargo-modules) of the `ra_ap_hir` crate that would benefit from being able to more efficiently query for all impls of a given module (instead of having to query for its parent crate's impls and then filtering on `impl_hir.module(db) == module_hir`).
I have the suspicion that the code as is won't quite work for file-level modules, since those don't have a block, afaict, but with all the crate-rename and version shenanigans around the `ra_ap_` release process I haven't yet been able to figure out how to patch a `ra_ap_` dependency from a third-party crate (let alone how to test this from inside of `rust-analyzer`), so haven't been able to actually run and test this. 😔
Any hints on how to make this (i.e. the code itself, as well as the testing) work are more than welcome!
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