v0.3.1: Improve efficiency by avoiding explicit whitespace rows

Previous versions used explicit zeroed rows corresponding to whitespace tokens in spaCy. This required duplication and a number of assignments into the transformer output, which was inefficient.

Instead, whitespace tokens are now regarded as not aligning to any wordpiece tokens. If you do doc._.trf_data[i] where i is the index of a whitespace token, you'll receive an array of shape (0, n) where n is the output dimension. This is handled in Thinc's pooling operations, so the change doesn't require any update to models consuming the trf_data.