Add CBOR output to rustdoc. Write a rustdoc server.

[mahkoh]

The current rustdoc html output is huge (e.g. 22M for https://github.com/mahkoh/posix.rs/) and the json output is still not compact enough (3.3M.)

Add CBOR output to rustdoc and write a server that dynamically generates the requested pages from this output and handles searching.

[tomjakubowski]

Couldn't (shouldn't?) this be done out of tree? JSON can already be converted to CBOR with existing tools and I'm somewhat uneasy about distributing an entire web server implementation with Rust, even if it isn't publicly exported in the std crate.

[retep998]

Having to wait for rustdoc to output tens of thousands of files takes a non-trivial amount of time. Outputting to a format designed for a web server in the first place would save a significant amount of time. Having it out of tree is fine, I just don't want to deal with rustdoc outputting so many files.

[tomjakubowski]

Have you profiled rustdoc and seen that it spends significant amounts of its time on converting its output into HTML? It seems likely to me that rustdoc, like a normal -O0 rustc invocation, spends most of its time on the analysis phase.

[huonw]

@tomjakubowski I suspect the IO overhead of creating thousands of files may be significant too.