change region_map to MapExpr so that it can be fusioned by optimization
change comment on map
Merge branch 'master' of github.com:spartan-array/spartan
fix region_map fn_kw containing Expr error
fix parakeet expr do not have fn attr error
Merge pull request #177 from MaggieQi/master

change region_map to MapExpr so that it can be fusioned by optimization