In Rust, it is UB if a slice is mutated while borrowed except through
the slice itself, and it is also UB if a mutable slice is read while
borrowed. The op macro allows borrowing an `ArrayBuffer{,View}` as a
memory slice for the duration of an op, but this is not sound for async
ops, since the `ArrayBuffer` could be accessed from JS during the await
points. This PR therefore disallows such automatic borrowing only for
async ops.
Co-authored-by: Divy Srivastava <dj.srivastava23@gmail.com>
