rustdoc: Show macros in documentation

Any macro tagged with #[macro_export] will be showed in the documentation for
that module. This also documents all the existing macros inside of std::macros.

Closes #3163
cc #5605
Closes #9954

src/libstd/lib.rs

@@ -77,7 +77,7 @@
 #[cfg(test)] pub use ops = realstd::ops;
 #[cfg(test)] pub use cmp = realstd::cmp;
 
-mod macros;
+pub mod macros;
 
 mod rtdeps;