If a reexport comes from a non-public module, then the documentation for the
reexport will be inlined into the module that exports it, but if the reexport is
targeted at a public type (like the prelude), then it is not inlined but rather
hyperlinked.
This adds support for the `--test` flag to rustdoc which will parse a crate,
extract all code examples in doc comments, and then run each test in the
extra::test driver.
This slurps up everything inside of an 'extern' block into the enclosing module
in order to document them. The documentation must be on the items themselves,
and they'll show up next to everything else on the module index pages.
Closes#5953
Removes old rustdoc, moves rustdoc_ng into its place instead (plus drops the _ng
suffix). Also shreds all reference to rustdoc_ng from the Makefile rules.
