update/insert columns -> assign

[MarcoGorelli]

closes #235

I think I made a mistake in splitting "set_column" into "insert_column" and "update_column":

1. first, because having tried using the Standard to write code a while now, it's really tedious to have to distinguish them
2. because I don't think any existing dataframe library has this distinction
3. a users has brought this up too (set_column_by_name/ set_column? #235)

In keeping with the goal of making the standard feel familiar, let's look at what existing dataframe libraries do:

• pandas and its clones: either __setitem__ (which the consortium doesn't want) or DataFrame.assign
• pyspark / polars: DataFrame.with_columns
• ibis: DataFrame.mutate
• datatable: update / extend

I don't really mind which one we go with. I'm suggesting with_columns here as I think it's probably the most explicit

[MarcoGorelli]

approved in today's call (but consensus on assign instead of with_columns)