Broadcast binary ops with device scalars (#502)

* implement broadcasting array binary ops with device scalars

* test broadcasting array binary ops with device scalars

* re-add some asserts (better to be safe)

* be explicit in error msg

op -> operator

Co-authored-by: Andreas Klöckner <inform@tiker.net>

* array binops: include asserts on out.shape as well

* set default args more elegantly

* Array shape checks: save shapes in temporaries

Co-authored-by: Andreas Klöckner <inform@tiker.net>