HirIdification: add key HirId methods
This is another PR in a series dedicated to `HirId`-ification, i.e. deprecating `ast::NodeId`s after the AST > HIR lowering process. The bigger proof of concept can be seen in #57578.
**Phase 2**: add key `HirId` methods mirroring the `NodeId` ones.
These should be counterparts of the most widely used `Hir` methods using `NodeId`s. Note that this expands `hir::map::Definitions` with an additional `hir_to_def_index` map (with the intention of later removing `node_to_def_index`).
As a bonus there is also a small cleanup commit removing unnecessary calls to `node_to_hir_id` where `HirId` is already available.
r? @Zoxc
Cc @varkor
4314dbaa76