Fix ecma visitor for function expressions with name.

To fix issue #27

Make the Getter and Setter visitors also handle the visiting of the
related FunctionExprNode, not rely on the visit_FunctionExprNode, which
can accordingly be changed to also include "function" before its name.

To DRY things up, move the common bit of all the Function-visiting
methods to #function_params_and_body helper method.

Also add a test case.