reduce the amount of panics in `{TokenStream, Literal}::from_str` calls
*[View all comments](https://triagebot.infra.rust-lang.org/gh-comments/rust-lang/rust/pull/147859)*
Before this PR, calling `TokenStream::from_str` or `Literal::from_str` with an invalid argument would always cause a compile error, even if the `TokenStream` is not used afterwards at all.
This PR changes this so it returns a `LexError` instead in some cases.
This is very theoretically a breaking change, but the doc comment on the impl already says
```
/// NOTE: some errors may cause panics instead of returning `LexError`. We reserve the right to
/// change these errors into `LexError`s later.
```
Fixes some cases of rust-lang/rust#58736.
Add new `byte_value` and `char_value` methods to `proc_macro::Literal`
Part of https://github.com/rust-lang/rust/issues/136652.
It adds two more methods to get unescaped `u8` and `char` from `proc_macro::Literal`.
r? @Amanieu
unstable proc_macro tracked::* rename/restructure
Picking up what should be the uncontroversial part of rust-lang/rust#87173 (closed due to inactivity over two years ago).
Part of rust-lang/rust#99515.
- move `proc_macro::tracked_env::var` to `proc_macro::tracked::env_var`
- move `proc_macro::tracked_path::path` to `proc_macro::tracked::path`
- change the argument of `proc_macro::tracked::path` from `AsRef<str>` to `AsRef<Path>`.
Stabilize `maybe_uninit_write_slice`
Stabilize feature `maybe_uninit_write_slice` (closes https://github.com/rust-lang/rust/issues/79995).
Note that this also const-stabilizes `<[MaybeUninit<_>]>::write_copy_of_slice`. That method depends on `<[_]>::copy_from_slice`, which is already const-stable, and `<[MaybeUninit<_>]>::assume_init_mut` which is now also stable.
cyrgani