Fix for generation of the block statement where the expression of an assignments was accidently visited twice
Added utility functionality to check whether a given expression is the initializing expression in an assignment
Added a 'NodeInfo' class used for wrapping any unsupported node, i.e. a VDM node that cannot be generated, and optionally the reason why it is not supported. Whether a VDM node can be generated may depend on the context.
Disallowed certain uses of the let-def expression that would reveal a bug with the code generator