Feature: Strided convolutions

Something like this would be amazing (I can't figure out how to do it efficiently in accelerate proper)
``` haskell
stridedConvolution
:: Exp sh --stride
-> Exp sh --padding?
-> (Exp a -> Exp b -> Exp c) --multiply
-> (Exp c -> Exp c -> Exp d) --add
-> Array sh b --kernel
-> Array sh a --source
-> Array sh d
```