Changelog for haskell-gi-base-0.20.1

Add Data.GI.Base.CallStack, abstracting (and backporting to the extent possible) the HasCallStack constraint present in newer GHCs. Using this, we now include callstacks pervasively in the generated code.
Improve the WrappedPtr implementation.
Deprecate nulltoNothing, it is better to simply fix the overrides when necessary.
Make the semantics of GObject ownership transfer closer to those used by the Python bindings.