Add extension to ScopedDisposable for SerialDisposable

[sirnacnud]

This adds an extension on ScopedDisposable where the inner is a SerialDisposable. This allows you to easily update the inner of the SerialDisposable wrapped in the ScopedDisposable.

Example:

let scopedDisposable = ScopedDisposable(SerialDisposable())
scopedDisposable.inner = someDisposble

Checklist

• [x] Updated CHANGELOG.md.