Positional naming/renaming of dataset columns

[respatialized]

I was working with a dataset that had repeated column names and the existing map-based methods made it tricky to rename the columns to avoid the duplicates. I was thinking that positional assignment of column names might be both a solution to this particular problem and a more generally useful capability for tech.ml.dataset. On Zulip, it was suggested that I open an issue here about this.

If possible, I'd like to work up a PR for this. It could either be a new function name-columns, or an extension to rename-columns that behaves differently if a vector is passed in. I have no opinion one way or another about which is preferable.

[cnuernber]

Both are good options - I would like to stick with rename-columns to avoid another symbol. If we go with a vector approach the only requirement I see is that the vector must be complete - it must have a name for every column. This is not currently true for the map approach.

[cnuernber]

Fixed by #264