Feature Req: Separable Convolution

[torridgristle]

The filters are made by multiplying against its flipped copy, so it should work fine if it was kept as, for example, 1x7 instead of 7x7. Then conv2d twice with the second conv2d using the weights after swapping the width and height dimensions.

I'm uncertain if this provides much of an improvement for a size of 3, but as the filter size grows it should be faster due to the number of reads increasing linearly as the width multiplied by two instead of exponentially as the width squared.

Edit: Sorry for the closed / reopen notifications, I thought I did something wrong when trying this again recently.