Add top-level `args` attribute for use with top-level map/try_map

In order to pass arguments to the input being passed to a top-level map function this is necessary.

```rust
#[derive_binread]
#[br(import { b: u64 })]
struct SomeOtherStruct {
// ...
}

#[derive_binread]
#[br(import { a: u32, b: u64 })]
#[br(args { b })]
#[br(map = |x: SomeOtherStruct| SomeStruct {/* ... */} )]
struct SomeStruct {
// ...
}
```

Start by tracing the top-level map/try_map handling and the derive_binread import and args syntax shown in the issue. Determine how the top-level args are passed to the mapped input, then add coverage for the example using b; done means the example compiles and the argument reaches SomeOtherStruct.

Written by the indexing model from the issue text.