Tweaks to deconstruction and reconstruction

[krlmlr]

• [ ] dm_deconstruct() generates the code to extract table objects and assign them to variables
• [ ] Implement nest(), should add/update a new primary key
• [ ] Implement nest_join()
• [ ] $ ~~and [[~~ return a dm_keyed_tbl, skip failing tests for now
• [ ] Use keyed = TRUE in more places
• [ ] Roundtrip for dm(new_table, my_dm)
• [ ] Roundtrip for dm(new_table, my_dm, my_dm_2)
• [ ] Roundtrip for dm(dm_get_tables(my_dm))
• [ ] Fix test failures
• [ ] Implement unnest() (in the case if data is a keyed tbl)
• [ ] Implement comprehensive tests on all dplyr, tidyr and other verbs

Follow-up to #1187.