Add `align_size_to` field-level attribute

[widberg]

This PR rounds off the pad_/align_ attribute symmetry by adding the counterpart to pad_size_to. I would especially appreciate any feedback on the documentation. In the future, I intend to pick up https://github.com/jam1garner/binrw/pull/250 and add both _size_to attributes at the struct/enum-level, if nobody beats me to it.