Add an API to get associated items

[celinval]

I would like to retrieve all associated items of a definition. Similar to CrateDefType, this could be provided via a trait CrateDefItems that return a list / map of all associated items.

Something similar to https://github.com/rust-lang/rust/blob/5affbb17153bc69a9d5d8d2faa4e399a014a211e/compiler/rustc_ty_utils/src/assoc.rs#L84.

[makai410]

@rustbot claim