Add unreleased section to CHANGELOG post 0.51.1 release
Add unreleased section to CHANGELOG post 0.51.1 release

jemc/ponyc,ponylang/ponyc,jemc/ponyc,dipinhora/ponyc,ponylang/ponyc,dipinhora/ponyc,jemc/ponyc,dipinhora/ponyc,ponylang/ponyc