rustdoc: Inline reexported items if possible

[lifthrasiir]

Fixes #13414 and possibly #13045. Some examples to look at: std::io, std::prelude, libc. (Sorry, no tests yet!)

There are some known exceptions to inlining (e.g. statics), so they are preserved to the reexport section as much as possible. It would be desirable to remove such exceptions in the future.

Some mandatory screenshots

From std::io:

From std::prelude (this would completely inline every item except for std::gc::GC, which is a static):

[brson]

This does not seem to be what #13414 is asking for. From what I can tell, that bug is asking that, in the documentation at the original location list the other reexports that this type is known as.

Regardless, this change is interesting. I was worried about doing this because it might make it look like there are multiple different types with the same name, but adding (Reexport) makes it obvious there's something else going on.

Is there any link from the reexport back to the original?

[lifthrasiir]

@brson Ah, well, I seemed to misread #13414. That would require a different machinery (namely, cache.reexports?) so it requires separate work.

Is there any link from the reexport back to the original?

There is no separate page for the reexported item anyway, so it always links to the original.

[lifthrasiir]

Fixed a commit message.

[alexcrichton]

Closing due to inactivity, but feel free to reopen with a rebase!