rustdoc should support hyperlinking between crates

[alexcrichton]

This is obviously highly desirable. This is also fairly tricky. An ideal solution would have these properties:

• Only one person has to specify that libstd/libextra are on the web at a specific URL
• I should be able to look at all docs without the web.
• I should be able to hyperlink to rustpkg libs just as easily as libstd/libextra.
• Linking across crates should have all the fancy typarams/lifetime bounds that linking intra-crate has.

I haven't quite fleshed out a good solution to this yes, so I'm opening this bug to track progress.