Generate rustdoc output instead of naturaldocs in build

[graydon]

This requires getting the output from rustdoc to a credible enough state that we can stand looking at its output instead of naturaldocs, rewriting the existing naturaldocs comments to rustdoc attributes, and adding some makefile logic to do the deed.

[graydon]

Despite heroic effort, punting on this for 0.1, doesn't seem worth blocking on. We'll change the website over when we have rustdoc sufficiently mature.

[brson]

Done.