Move the unstable-api dump tool to the main rust repo

[albertlarsan68]

This will allow to use the json rustdoc output, thus having the macros expanded for free, and there is already code for finding this in tidy and unstable-book-gen.

[m-ou-se]

Yeah it'd be cool to have a unstable-api tool that's based on rustc/rustdoc, rather than syn. :)

[albertlarsan68]

@rustbot claim

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