Reorganize Data.Sequence

Some things are arranged a bit oddly. Notably, the helper functions `deepL`, `deepR`, `pullL`, and `pullR` are currently located very far from the functions that actually use them. They're also not documented as well as one might like (and the `pull` names are not exactly intuitive). I also have the nagging feeling that things might be easier to understand if `pullL` and `pullR` are inlined into `deepL` and `deepR`.