rustdoc search should not use `str` as keyword for searching for structs

[huonw]

E.g. a search for str::raw becomes a search for structs called :raw. This makes it impossible to search more for things inside the str precisely.

This would be fixed by expanding str to struct.

[Gankra]

struct is already a keyword, str is just a synonym that gets expanded to it. So just nix the synonym, then?