A very simple macro that clones a list of variables before calling an expression

Based on this tweet: https://twitter.com/untitaker/status/1299812136202493953

```rust use clone_block::clone; use std::thread;

let foo = "foo".to_string();

let thread = thread::spawn( clone!(foo; move || { let foobar = format!("{}bar", foo); foobar }) );

let foobar = thread.join(); let foobaz = format!("{}baz", foo); ```

License

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