Consider replacing `as_list_col`, `names_sep` with `dplyr` behavior

[brookslogan]

We might want to make things more dplyr-like by not prefixing to begin with, eliminating as_list_col and let users just wrap their output in a list if they want the corresponding output column to be a list, and eliminating names_sep. (Might avoid confusion leading to filing #229.) Relates to #240.

Not sure if we should treat this as a P1 instead, as this would be a breaking change for users that do rely on the prefixed-unnest behavior.

[brookslogan]

Another point of confusion this could avoid: if we provide a computation outputting atomic vectors, slides with as_list_col=TRUE won't actually output a list col.

[dshemetov]

These were removed in #477