Annotation: Default arguments to null

Annotations without arguments are a pretty common case.
CeylonExpressionTransformer: Improve annotations

For annotations without arguments – the common case – the empty
arguments are no longer shown.
CeylonExpressionTransformer: Improve string literals

For non-verbatim string literals, the ‘isVerbatim’ flag is no longer
shown.
CeylonExpressionTransformer: Fix TypeNameWithTypeArguments
completeCompilationUnit: Add function definition
completeCompilationUnit: Add class definition