merge `as_local_hir_id` with `local_def_id_to_hir_id`
`as_local_hir_id` was defined as just calling `local_def_id_to_hir_id` and I think that having two different ways to call the same method is somewhat confusing.
Don't really care about which of these 2 methods we want to keep.
Does this require an MCP, considering that these methods are fairly frequently used?
- Move the type parameter from `encode` and `decode` methods to
the trait.
- Remove `UseSpecialized(En|De)codable` traits.
- Remove blanket impls for references.
- Add `RefDecodable` trait to allow deserializing to arena-allocated
references safely.
- Remove ability to (de)serialize HIR.
- Create proc-macros `(Ty)?(En|De)codable` to help implement these new
traits.
Some helpers functions have been introduced to deal with (buggy) cases
where either a `NodeId` or a `DefId` do not have a corresponding `HirId`.
Those cases are tracked in issue #71104.
