Refactor XRef in obj.js

- remove unused "if (e instanceof Stream)" in XRef_fetch
- split XRef_fetch into XRef_fetchUncompressed and XRef_fetchCompressed
This explains the actual code flow better
- add line breaks after functions
Merge branch 'refactor/obj.js/2' of https://github.com/bthorben/pdf.js into refactor/obj.js/2