rustdoc: wrap Type with Box instead of Generics

[notriddle]

When these Box<Generics> types were introduced, Generics was made with Vec and much larger. Now that it's made with ThinVec, Type is bigger and should be boxed instead.

[rustbot]

r? @jsha

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[rustbot]

Some changes occurred in src/librustdoc/clean/types.rs

cc @camelid

[rustbot]

Some changes occurred in src/librustdoc/clean/types.rs

cc @camelid

[fmease]

Thanks, much better!

r? fmease @bors r+

[bors]

📌 Commit f1a1ef6 has been approved by fmease

It is now in the queue for this repository.

[fmease]

Might have some perf implications since there's now one more level of indirection for accessing the type of a constant but otoh we also remove one level for generics and both fields are accessed the same amount of times. Therefore I'm not sure if it should be rollup=never 🤔