Organize our collation utils

[SalmanMohammadi]

see #1005 for some context. From @ebsmothers (and @joecummings)@

I don't love our collate utilities rn. In an ideal world I want two simple abstractions: right_padded_collate and left_padded_collate, and any further padding built up on top of that. I don't think this version should exist at all (especially with its heavy dictionary usage)