The reason the ACE editor was failing to load the rust syntax
highlighting is because the syntax highlighting was being created
*after* the editor was created. If the editor is created first, then ACE
tries to load `ace/mode/rust`. Since it isn't already defined, it tried
to compute the URL and load it manually. However, since the URLs now
have a hash in it (via https://github.com/rust-lang/mdBook/pull/1368),
it was unable to load.
The solution here is to make sure `ace/mode/rust` is defined before
creating the editors. Then ACE knows that it can just load the module
directly instead of trying to fetch it from the server.
Fixes https://github.com/rust-lang/mdBook/issues/2700
