fix reading of bclosures

Reading bclosures has been broken since commit 991bc3ab, where the syntax of writing bytecodes was changed. Since then, the syntax for bytecodes also includes a redundant field for the lexical environment, which is always nil. @jackdaniel, do you have any idea why the change in commit 991bc3ab was made? Because I think that the old syntax makes more sense and we should switch back to it if possible.