Change keypath `BindingTarget.init` to solve source breaking change in SE-0481

[mluisbrown]

Fixes: #889

SE-0481 would break the current version of the BindingTarget.init that takes a keypath. Changing the keypath type from WritableKeyPath to ReferenceWritableKeyPath fixes this, and is probably what the type should always have been, since Object will always be a reference type.

Despite the signature of the initalizer changing, this is unlikely to break existing code using it, as the keypath is almost always passed using keypath notation (eg : \.property) and not using an explicit keypath type.

Checklist

• [x] Updated CHANGELOG.md.