Add rustdoc_ng
Slight restructure of rustdoc_ng
Finish addition of rustdoc_ng to Makefiles.
auto merge of #8540 : cmr/rust/rustdoc_ng, r=pnkfelix

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?