Extract `rustc_hir` out of `rustc`
The new crate contains:
```rust
pub mod def;
pub mod def_id;
mod hir;
pub mod hir_id;
pub mod itemlikevisit;
pub mod pat_util;
pub mod print;
mod stable_hash_impls;
pub use hir::*;
pub use hir_id::*;
pub use stable_hash_impls::HashStableContext;
```
Remains to be done in follow-up PRs:
- Move `rustc::hir::map` into `rustc_hir_map` -- this has to be a separate crate due to the `dep_graph` (blocked on https://github.com/rust-lang/rust/pull/67761).
- Move references to `rustc::hir` to `rustc_hir` where possible.
cc https://github.com/rust-lang/rust/issues/65031
r? @Zoxc
7494250106