Issue Description:

The `Data` custom deallocator is currently defined as taking the ptr and the length:

case custom((UnsafeMutableRawPointer, Int) -> Swift.Void)

This is problematic when using the feature to wrap an existing buffer (presumably the most common use case), for example: NIO.ByteBuffer.readData

This thing has to do an allocation to capture the reference to the underlying storage. To avoid that, it would be great to have a context pointer in the custom deallocator, like so:

case customWithContext((UnsafeMutableRawPointer, Int, UnsafeRawPointer) -> Swift.Void)

This way the dealloc closure would be context free, no extra alloc would be necessary.