Improve item transformation with special characters

[poliorcetics]

For example macros can be suffixed with !, functions and methods with () and primitives can be prefixed primitive@.

See this PR for some background.

[jyn514]

I wrote this up already, so pasting here:

Currently, intraconv will replace all items with only their name:

-//! [instrument]: attr.instrument.html

This leads to ambiguity if the same name is present in multiple namespaces:

warning: `instrument` is both a function and an attribute macro
 --> tracing-attributes/src/lib.rs:5:23
  |
5 | //! [`#[instrument]`][instrument] procedural macro attribute.
  |                       ^^^^^^^^^^ ambiguous link

It would be great to add a disambiguator so these didn't have to be fixed manually. The following transformations should work:

"mod" => "mod@", // this one needs to be different because of primitives
"struct" | "enum" | "trait" | "union" | "type" |  => "type@", // technically these should use struct@, etc. but it's non-idiomatic to use a lowercase name so it shouldn't come up in practice
"const" | "static" | "value" => "value@",
"fn" => "()",
"derive" | "attr" => "macro@",
"macro" => "!",

There might be more I haven't thought of but this should hit the common cases. Places to look at in rustdoc:
https://github.com/rust-lang/rust/blob/master/src/librustdoc/passes/collect_intra_doc_links.rs#L1281-L1320
https://github.com/rust-lang/rust/blob/master/src/librustdoc/formats/item_type.rs#L127-L156

[poliorcetics]

I just pushed a fix for it, don't hesitate to open a new issue/reopen the existing one if you find failing cases.