Rustdoc search should support space-separated queries (in additon to `::`-separated)

[thomcc]

I often try to do searches on rustdoc as foo bar rather than foo::bar, since it's easier to type and to me it feels more natural, especially in situations where the path isn't exact (e.g. see example 2 below).

Unfortunately, this pretty much never works, and it would be nice if it did. Concrete examples:

1. I'd like it if searching vec truncate brought up links to std::vec::Vec::trunctate and such, the way searching vec::truncate does.

2. I'd like it if arch mm_cvt brought up the list of _mm_cvt functions in core::arch, as searching arch::mm_cvt does

   (Note that in this case, the :: path isn't really actually a thing that appears anywhere in the results, which is part of why I think spaces for this can be more natural)

3. Ideally when spaces are used, the order wouldn't be important. e.g. mm_cvt x86_64 would work even though mm_cvt::x86_64 doesn't — it would ideally something like this.

   That said, this might be better addressed later, I can't really make heads or tales of most of what the code is doing, so I don't know how big of a deal it is.

I think this is a matter of replacing some of the use of "::" in this file, but I'm not completely sure all the changes that are needed.

[nerandell]

@rustbot claim

[Dylan-DPC]

fixed by #88268