[R] Add str() methods to objects

[asfimport]

Apparently this will make the RStudio IDE show useful things in the environment panel. Probably this is most useful for Table, RecordBatch, and Dataset (and maybe Schema, which would look similar).

Reporter: Neal Richardson / @nealrichardson

_{Note: This issue was originally created as ARROW-8718. Please see the migration documentation for further details.}

[asfimport]

Neal Richardson / @nealrichardson:
We can add the methods, but the desired IDE behavior is blocked. See rstudio/rstudio#7147.

[thisisnic]

This issue hasn't had activity in a long time. If it's still being worked on, please leave a comment. Otherwise, it will be closed on 23rd June.

Labelled Status: Stale-Warning for tracking.