Parse static closure coroutine syntax

- https://github.com/rust-lang/rust/issues/43122

Example: https://github.com/rust-lang/rust/blob/a6050b7ea848e25289bf097c606e0611c58ef4f3/tests/ui/coroutine/size-moved-locals.rs

```rust
#[coroutine] static || { yield; };
```

Start by comparing the requested `#[coroutine] static || { yield; };` syntax with Rust issue #43122 and the linked `tests/ui/coroutine/size-moved-locals.rs` example. Locate syn's coroutine and closure parsing entry points; done means this static coroutine closure parses successfully without regressing existing syntax.

Written by the indexing model from the issue text.