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
- change the document to conform to current StyleGuide
Merge pull request #4348 from bthorben/refactor/obj.js/2

Refactor XRef in obj.js