rustdoc-search: add support for nested generics
This change allows `search.js` to parse nested generics (which look `Like<This<Example>>`) and match them. It maintains the existing "bag semantics", so that the order of type parameters is ignored but the number is required to be greater than or equal to what's in the query.
For example, a function with the signature `fn read_all(&mut self: impl Read) -> Result<Vec<u8>, Error>` will match these queries:
* `Read -> Result<Vec<u8>, Error>`
* `Read -> Result<Error, Vec>`
* `Read -> Result<Vec<u8>>`
But it *does not* match `Result<Vec, u8>` or `Result<u8<Vec>>`.
3312a3053b