Ability to mark nodes as implicitly safe

[m-vo]

What is considered safe or not in the AST, is currently hardcoded in the SafeAnalysisNodeVisitor. For instance, a node that is an instance of ConstantExpression, is considered safe in all contexts. So when I'm creating my own expressions (that are safe by definition), I need to extend from ConstantExpression for the safe analysis to still work and the escaper node visitor not affecting the output.

In c44bd1f we're suddenly making assumptions about the class from outside. This will break child classes: In the above example a PrintNode wrapping a FooConstantExpression now gets converted to a TextNode ignoring the custom implementation.

How to deal with this? IMHO the ideal way would be to mark the ConstantExpression as @final and introduce an interface/flag for nodes, that marks them as safe in a certain or all contexts. Alternatively the optimizer should probably use get_class instead of instanceof.