Removed as many "```ignore" as possible.

Replaced by adding extra imports, adding hidden code (`# ...`), modifying
examples to be runnable (sorry Homura), specifying non-Rust code, and
converting to should_panic, no_run, or compile_fail.

Remaining "```ignore"s received an explanation why they are being ignored.

Author: kennytm

src/liballoc/boxed.rs

@@ -42,8 +42,10 @@
 //! Recursive structures must be boxed, because if the definition of `Cons`
 //! looked like this:
 //!
-//! ```rust,ignore
+//! ```compile_fail,E0072
+//! # enum List<T> {
 //! Cons(T, List<T>),
+//! # }
 //! ```
 //!
 //! It wouldn't work. This is because the size of a `List` depends on how many

src/liballoc/fmt.rs

@@ -230,7 +230,7 @@
 //! There are a number of related macros in the `format!` family. The ones that
 //! are currently implemented are:
 //!
-//! ```ignore
+//! ```ignore (only-for-syntax-highlight)
 //! format!      // described above
 //! write!       // first argument is a &mut io::Write, the destination
 //! writeln!     // same as write but appends a newline