Add ide-assist: generate_struct

Adds a stub struct with a definition matching the expression under the cursor.

Example
---
```rust
fn foo() {
    let bar = Bar$0 { x: 2, y: 3, force: true };
}
```
->
```rust
fn foo() {
    let bar = Bar { x: 2, y: 3, force: true };
}

struct Bar { x: i32, y: i32, force: bool }
```

Author: A4-Tacks