Simplify simple return statements.

I noticed a lexer of mine had a lot of the following generated code:
```
switch($avoiding_name_collisions) {
case 0:/* ignore */
case 1:return 13
break;
case 2:return 16
break;
case 3:return 25
break;
case 4:return 278
break;
case 5:return 279
break;
// …
case 120:console.log(yy_.yytext);
break;
}
```

**This pull requests simplifies this generated code into:**
```
switch($avoiding_name_collisions) {
case 0:/* ignore */
break;
case 120:console.log(yy_.yytext);
break;
default:
  return $case_helper[$avoiding_name_collisions]
}
```
That would invoke creation of a new object every time (of which only one value would be used). I purposely put it in a surrounding anonymous function, so it is created only once and subsequently reused.

Another way to achieve the same thing is to split the code in common code and module code, as I did with Jison.