10539: Add "generate delegate methods" assist r=Veykril a=yoshuawuyts
_Co-authored with `@rylev_.`
This patch adds a new assist: "generate delegate method" which creates a method that calls to a method defined on an inner field. Delegation is common when authoring newtypes, and having IDE support for this is the best way we can make this easier to author in Rust, bar adding language-level support for it. Thanks!
Closes#5944.
## Example
__before__
```rust
struct Age(u8);
impl Age {
fn age(&self) -> u8 {
self.0
}
}
struct Person {
ag$0e: Age,
}
```
__after__
```rust
struct Age(u8);
impl Age {
fn age(&self) -> u8 {
self.0
}
}
struct Person {
age: Age,
}
impl Person {
$0fn age(&self) -> u8 {
self.age.age()
}
}
```
Co-authored-by: Ryan Levick <me@ryanlevick.com>
Co-authored-by: Yoshua Wuyts <yoshuawuyts@gmail.com>
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