refactor(AST): introduce AstVisitor trait

The AST-Visitor trait allows generic visiting of AST-nodes. A default
Walking behavior is implemented for each AstStatement but it can be
altered by any implementor. When overriding a visit_XXX method, the
implementation can decide to continue with the default walking behavior
(by calling the walk function on the passed AstStatement-Struct, to skip it,
or to continue with an alternative walking bahavior.

compiler/plc_ast/src/lib.rs

@@ -7,3 +7,4 @@ pub mod control_statements;
 pub mod literals;
 mod pre_processor;
 pub mod provider;
+pub mod visitor;