repair macro docs

[jbclements]

In f1ad425, I changed the handling
of macros, to prevent macro invocations from occurring in fully expanded
source. Instead, I added a side table. It contained only the
spans of the macros, because this was the only information required
in order to make macro export work.

However, librustdoc was also affected by this change, since it
extracts macro information in a similar way. As a result of the earlier
change, exported macros were no longer documented.

In order to repair this, I've adjusted the side table to contain whole
items, rather than just the spans.

Note that the resulting macro documentation now appears at the
top level of the crate.

Fixes #15645

[emberian]

Appearing at the toplevel isn't as nice since std::macros will be floating. But until import/export is worked out, it's probably fine...

[lilyball]

Note that the resulting macro documentation now appears at the
top level of the crate.

Won't that still break the std::macros documentation?

[emberian]

r+ retracted. @jbclements travis failed with a legit fold_mac error in stdtest

[sfackler]

It seems like the easy solution is to just tack on #[doc(hidden)] to std::macros.

[jbclements]

ha! funny! hoist by my own petard. These are the checks that I added to make sure that no macros are left in the crate after expansion. But this patch does stow those macros in the crate. I'm just going to tweak fold_crate, a bit.