Add rustdoc_ng
Finish addition of rustdoc_ng to Makefiles.
Update rustdoc_ng to new ast naming
Update rustdoc_ng to syntax and metadata changes
rustdoc_ng: add licenses and issue numbers to appease tidy
auto merge of #8540 : cmr/rust/rustdoc_ng, r=catamorphism

This doesn't enable it by default yet, or include the frontend. But the tool
is pretty much done, and it'd be nice to stave off bitrot.

I couldn't get it to actually build with the Makefile though. I don't really
understand what is going on in that makefile at all. Could @graydon or
@pnkfelix lend a hand?