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

[jam1garner]

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

#[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 {
  // ...
}