rustdoc/html: emphasize the crate part of the full path.

If you browse to, say, http://doc.rust-lang.org/libc/types/os/common/posix01/struct.timeval.html , you will see the "location" window showing `libc::types::os::common::posix01`.  The first element points to a crate and others point modules.  This patch adds the bold attribute to the first (ie. crate) element so that it stands out more.