Allow generic parameters in intra-doc links

The contents of the generics will be mostly ignored (except for warning
if fully-qualified syntax is used, which is currently unsupported in
intra-doc links - see issue #74563).

* Allow links like `Vec<T>`, `Result<T, E>`, and `Option<Box<T>>`
* Allow links like `Vec::<T>::new()`
* Warn on
  * Unbalanced angle brackets (e.g. `Vec<T` or `Vec<T>>`)
  * Missing type to apply generics to (`<T>` or `<Box<T>>`)
  * Use of fully-qualified syntax (`<Vec as IntoIterator>::into_iter`)
  * Invalid path separator (`Vec:<T>:new`)
  * Too many angle brackets (`Vec<<T>>`)
  * Empty angle brackets (`Vec<>`)

Note that this implementation *does* allow some constructs that aren't
valid in the actual Rust syntax, for example `Box::<T>new()`. That may
not be supported in rustdoc in the future; it is an implementation
detail.

src/librustdoc/lib.rs

@@ -3,11 +3,13 @@
     html_playground_url = "https://play.rust-lang.org/"
 )]
 #![feature(rustc_private)]
+#![feature(array_methods)]
 #![feature(box_patterns)]
 #![feature(box_syntax)]
 #![feature(in_band_lifetimes)]
 #![feature(nll)]
 #![feature(or_patterns)]
+#![feature(peekable_next_if)]
 #![feature(test)]
 #![feature(crate_visibility_modifier)]
 #![feature(never_type)]