Add repr(transparent) to hyper_context

[Manishearth]

https://github.com/hyperium/hyper/blob/1d4ff3597b8e76818c8553dbfa4234cf4208c958/src/ffi/task.rs#L504

This comment is incorrect, repr(Rust) provides no such guarantees.

(Also we probably should make most of these FFI types repr(C)))