rustc_feature: Separate "renamed" features from "removed" features

[nik-rev]

There are quite a few "renamed" features that are grouped in the same category as "removed".

So I've split them up into a separate renamed.rs file, and added a new category for "Renamed" features.

This has the following advantages:

• We now have a new error specifically for "renamed" features, which suggests replacing the old feature name with the new feature name, which means we can (probably) feel less bad about renaming them
• The tidy script is updated to check that the new feature names actually exist.
• My next PR: When renaming a feature, we don't just instantly break nightly users - we can have a grace period (e.g., 3 versions) where the old name is still recognized, but a warning is emitted

[rustbot]

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