Fix some issues with the rust-lang/rust upstream.

[ehuss]

This fixes some more issues trying to get mdbook-spec to work in rust-lang/rust.

• Regex must be downgraded because memchr is pinned and we need to use older versions of regex because of that.
• Set the name of the preprocessor correctly (required for how it is integrated upstream).
• Added some elided lifetimes because the rust repo is finicky about using elided_lifetimes_in_paths.
• Don't assume rustdoc is in PATH, since when building it isn't there.