Use `ifNil:`, `ifNotNil:` and `ifNil:ifNotNil:` instead of `isNil` and`ifTrue/ifFalse` combinations
metaclasses use Association as a class binding. We should make asubclass of LiteralVariable so that we can implement a compatible API
- use LiteralVariable instead of dedicated ClassBinding (until we areforced to use a dedicated class we do not need it)