Detect `AString.clone()` when `&str` is needed and emit appropriate suggestion

[estebank]

On type mismatch with a suggestion to borrow in order to dereference a String into a &str, we should notice unnecessary .clone() calls:

error[E0308]: mismatched types
 --> src/main.rs:7:38
  |
7 |     assert_eq!(make_lipographic('e', passage.clone()), passage);
  |                                      ^^^^^^^^^^^^^^^
  |                                      |
  |                                      expected &str, found struct `std::string::String`
  |                                      help: consider borrowing here: `&passage.clone()`
  |
  = note: expected type `&str`
             found type `std::string::String`

and suggest their removal:

error[E0308]: mismatched types
 --> src/main.rs:7:38
  |
7 |     assert_eq!(make_lipographic('e', passage.clone()), passage);
  |                                      ^^^^^^^^^^^^^^^
  |                                      |
  |                                      expected &str, found struct `std::string::String`
  |                                      help: consider borrowing and avoiding the clone here: `&passage`
  |
  = note: expected type `&str`
             found type `std::string::String`

From https://thenewwazoo.github.io/clone.html