Rustdoc search should allow ` ` as a separator in addition to `::`

[thomcc]

If I search vec push in the stdlib docs, it brings me to this page which currently has no results. This is unfortunate, and I hit it probably at least once a day, and may not be alone.

Searching instead for vec::push works, but it would be nice if it were not necessary, and the delimiter could be as well.

[GuillaumeGomez]

Sounds acceptable. It'll need to go through FCP I think but otherwise I think it makes sense.