Fix Zip handling in type checker.
Fix Zip handling in type checker.

mrakgr/futhark,HIPERFIT/futhark,diku-dk/futhark,diku-dk/futhark,diku-dk/futhark,diku-dk/futhark,mrakgr/futhark,ihc/futhark,CulpaBS/wbBach,mrakgr/futhark,ihc/futhark,ihc/futhark,HIPERFIT/futhark,HIPERFIT/futhark,diku-dk/futhark