This commit adds support for unstable FFI
callbacks. A callback is registered using
the `Deno.UnsafeCallback` API.
The backing memory for the callback can
be disposed of using `Deno.UnsafeCallback#close`.
It is not safe to pass the callback after calling
close.
Callbacks from other than the isolate thread
are not supported.
Co-authored-by: Divy Srivastava <dj.srivastava23@gmail.com>
Co-authored-by: Bert Belder <bertbelder@gmail.com>
