`sink_` / `write_` methods should invisibly return the input data

[etiennebacher]

It is sometimes useful to return the input data as an invisible value. This is what arrow and readr do for example:

• https://arrow.apache.org/docs/r/reference/write_parquet.html?q=write_parquet#value
• https://readr.tidyverse.org/reference/write_delim.html#value

[eitsupi]

I think that makes sense.

I recall that some of the SQLContexts methods also had both side-effects and returns.