Use proc-macro to derive HashStable everywhere
Hello,
A second proc-macro is added to derive HashStable for crates librustc depends on.
This proc-macro `HashStable_Generic` (to bikeshed) allows to decouple code and strip much of librustc's boilerplate.
Still, two implementations `Span` and `TokenKind` require to be placed in librustc.
The latter only depends on the `bug` macro. Advise welcome on how to sever that link.
A trait `StableHasingContextLike` has been introduced at each crate root,
in order to handle those implementations which require librustc's very `StableHashingContext`.
This overall effort allowed to remove the `impl_stable_hash_for` macro.
Each commit passes the `x.py check`.
I still have to double check there was no change in the implementation.
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