Move markdown library out of librustdoc into it's own libmarkdown

[mneumann]

Rust ships with a Markdown implementation. By moving it into it's own library, it's less hidden to the programmer and other people can depend on it for their own projects.

I know that I can use it right now via rustdoc::html::markdown::Markdown, but this also means that I depend on the whole rustdoc library, not just markdown.

I think if we ship with markdown we should make it available as a first class library for everyone to use and depend on.

[flaper87]

I think it is a good idea to pull it out of rustdoc

[alexcrichton]

As the 1.0 release is focused on the API stability of the standard library, I don't think that this is going to happen in the mainline distribution. The shipped version of libmarkdown (if we create one), will be intentionally gated off with #![experimental]. We do have a number of libraries in this situation (see the lists on #18585), as well as a theoretical strategy for dealing with how to ship them.

Times have changed since this bug was created, however, and we primarily now have Cargo. I think for now we'd have a difficult time refactoring librustdoc to extract the Markdown implementation in any sort of a usable fashion, so I'm going to close this for now because we're able to have a high-quality markdown crate in the Cargo ecosystem rather than the standard library itself.

We can certainly investigate bringing in an excellent implementation for use by rustdoc as well!