Parse named argument syntax in parenthesized generic arguments

- https://github.com/rust-lang/rfcs/pull/3955
- https://github.com/rust-lang/rust/issues/158499

```rust
fn parse_my_data(
data: &str,
log: impl Fn(msg: String, priority: usize),
)
```

Start by reading RFC 3955 and rust-lang/rust issue 158499 to understand the named-argument syntax and its intended scope. Then locate the relevant syn parser entry point and add coverage for the shown parenthesized generic argument form; done means that syntax parses successfully without regressing existing forms.

Written by the indexing model from the issue text.