Validation framework

[ftomassetti]

Currently to validate the AST we write rules that needs to traverse the entire tree. If we write 20 rules, we traverse the tree 20 times. We could instead create a mechanism to register validators for different node types, traverse the tree once, and on each node invoke the validators. The validators would consist of a predicate and an issue generator. If the predicate is satisfied the issue generator is invoked and the issue added to the list.