This allows to avoid looking at HIR from borrowck.

check_doc_alias_value: get argument as Symbol to prevent needless string convertions

check_doc_attrs: don't alloc vec, iterate over slice. Vec introduced in #83149, but no perf run posted on merge

replace as_str() check with symbol check

get_single_str_from_tts: don't prealloc string

trivial string to str replace

LifetimeScopeForPath::NonElided use Vec<Symbol> instead of Vec<String>

AssertModuleSource use BTreeSet<Symbol> instead of BTreeSet<String>

CrateInfo.crate_name replace FxHashMap<CrateNum, String> with FxHashMap<CrateNum, Symbol>

This reverts commit 22ae20733515d710c1134600bc1e29cdd76f6b9b.

- 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.

For both `ast::Expr` and `hir::Expr`.