New lint: existing trait method is no longer unsafe

[obi1kenobi]

Major breaking change because existing impls of that trait must remove the unsafe from that method as well.

Example: (playground)

trait Example {
    fn method();
}

struct MyStruct;

impl Example for MyStruct {
    unsafe fn method() {}
}

This errors out:

error[E0053]: method `method` has an incompatible type for trait
 --> src/lib.rs:8:5
  |
8 |     unsafe fn method() {}
  |     ^^^^^^^^^^^^^^^^^^ expected normal fn, found unsafe fn
  |
note: type in trait
 --> src/lib.rs:2:5
  |
2 |     fn method();
  |     ^^^^^^^^^^^^
  = note: expected signature `fn()`
             found signature `unsafe fn()`

For more information about this error, try `rustc --explain E0053`.

Going in the opposite direction is breaking too: #605

[obi1kenobi]

Implemented in #612.