This is implemented, in addition to the ML-style one,
because Rust does it. If we don't, we'll never hear the end of it.
This commit also refactors some duplicate parts of the parser
into a dedicated function.
This makes sense, since the search index has the information in it,
and it's more useful for function signature searches since a
function signature search's item type is, by definition, some type
of function (there's more than one, but not very many).
