Add adjust return type assist

Hey folks, what do you think about adding above-mentioned assist? It could be useful when doing some refactors. If this makes sense, I can try to do it as this will boost my dev workflow. If not, feel free to close this issue.

silly example: If I change Ok(Foo) to Ok(Bar) I have to adjust return type by myself, but I'd like to quick fix it, if possible. I browsed through already existing assists and there is add_return_type but I can't see anything which suits my case.

struct Foo;
struct Bar;
fn foo() -> Foo {
  Foo // -> change to Bar
}

struct Foo;
struct Bar;
fn foo() -> Foo {
  Bar // offer quick fix action
}

Start by reviewing the existing add_return_type assist and other assist implementations; the issue does not name specific files or tests. Determine how an assist could detect a returned expression whose type differs from the declared return type, then define tests for the Foo/Bar example and verify that a quick fix offers to adjust the return type.