Get rid of the hir_owner query. This query was meant as a firewall between `hir_owner_nodes` which is supposed to change often, and the queries that only depend on the item signature. That firewall was inefficient, leaking the contents of the HIR body through `HirId`s. `hir_owner` incurs a significant cost, as we need to hash HIR twice in multiple modes. This PR proposes to remove it, and simplify the hashing scheme. For the future, `def_kind`, `def_span`... are much more efficient for incremental decoupling, and should be preferred.