Add a fix that inserts a missing method

[MatrixFrog]

Work in progress! Not ready for review yet.

Often I will call a method even though I know it doesn't exist yet, because I'm planning to add that method.

struct Dog {}

impl Dog {
  bark(&self) {}
}

fn main() {
  let dog = Dog {};
  dog.bark();
  dog.play_dead();  // errors because the method doesn't exist yet
}

This PR adds a fix that inserts a stub for the play_dead method.

• For now, the receiver type is &self but maybe we can figure out whether it should be self or &mut self instead?
• Return type is not added but it could be inferred if the calling code looks like let lhs: SomeType = dog.play_dead()
• Should the method body be todo!() or unimplemented!() or something else? What is the most consistent with how r-a does things elsewhere?
• Need to do more testing

[ChayimFriedman2]

Should the method body be todo!() or unimplemented!() or something else? What is the most consistent with how r-a does things elsewhere?

There is a config for what to insert in e.g. fill match arms, I think it's appropriate to use it here as well.