Add arrayDims function (does something else than the old one).
Add arrayDims function (does something else than the old one).

Express arrayRank in terms of it.

Also fixed some misleading documentation related to arrayRank.

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