O(log n) lookup of associated items by name
Resolves#68957, in which compile time is quadratic in the number of associated items. This PR makes name lookup use binary search instead of a linear scan to improve its asymptotic performance. As a result, the pathological case from that issue now runs in 8 seconds on my local machine, as opposed to many minutes on the current stable.
Currently, method resolution must do a linear scan through all associated items of a type to find one with a certain name. This PR changes the result of the `associated_items` query to a data structure that preserves the definition order of associated items (which is used, e.g., for the layout of trait object vtables) while adding an index of those items sorted by (unhygienic) name. When doing name lookup, we first find all items with the same `Symbol` using binary search, then run hygienic comparison to find the one we are looking for. Ideally, this would be implemented using an insertion-order preserving, hash-based multi-map, but one is not readily available.
Someone who is more familiar with identifier hygiene could probably make this better by auditing the uses of the `AssociatedItems` interface. My goal was to preserve the current behavior exactly, even if it seemed strange (I left at least one FIXME to this effect). For example, some places use comparison with `ident.modern()` and some places use `tcx.hygienic_eq` which requires the `DefId` of the containing `impl`. I don't know whether those approaches are equivalent or which one should be preferred.
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