Fixes layout parsing of operators

This PR tries to improve the current situation of layout parsing of operators, such that the bug highlighted in https://github.com/idris-lang/Idris-dev/issues/729#issuecomment-30588237 is fixed.

There are still some way to go on fixing the general layout parsing, but hopefully it will solve/improve upon https://github.com/idris-hackers/IdrisWeb/issues/7 in Idris-web.