Visitor for `FunctionDeclaration` AST node.

def visit_FunctionDeclaration(self, node):
        """Visitor for `FunctionDeclaration` AST node."""
        scope = self.memory.stack.current.current.copy()
        args = [parameter.variable.identifier.name for parameter in node.parameters]

        for i in islice(scope, len(args)):
            self.memory.stack.current.current.pop(i)
            self.memory[args[i]] = scope[i]

        return self.visit(node.body)