Suggest extern crate alloc; when using undeclared module alloc

[aDotInTheVoid]

Given the following code: link

use alloc::rc::Rc;

The current output is:

error[E0433]: failed to resolve: use of undeclared crate or module `alloc`
 --> src/lib.rs:2:5
  |
2 | use alloc::rc::Rc;
  |     ^^^^^ use of undeclared crate or module `alloc`

Ideally the output should look like:

error[E0433]: failed to resolve: use of undeclared crate or module `alloc`
 --> src/lib.rs:2:5
  |
2 | use alloc::rc::Rc;
  |     ^^^^^ use of undeclared crate or module `alloc`
help: add `extern crate alloc` to use the buildin `alloc` module