Parse attributes inside of tuple patterns

- https://github.com/rust-lang/rfcs/pull/3532

```rust
let (a, #[cfg(something)] b) = my_tuple;
let MyStruct(a, #[cfg(something)] b) = x;
```

Start with RFC 3532 and the two tuple-pattern examples in this issue. Trace how syn currently parses tuple and tuple-struct patterns, then add focused coverage for attributes in both forms; done means the examples parse successfully without regressing existing pattern parsing.

Written by the indexing model from the issue text.