Improve access to data.tables metadata - `tables()`, `columns()`

[jangorecki]

FR for functions to allow easier access to data.tables metadata and easier audit/programmatic use. Function tables could be improved, and new function columns could provide information on columns.

Follow up of @rsaporta old PR #845
Also related to #718

[MichaelChirico]

What do you have in mind for the columns function? It's not clear...

Also adding that there was some discussion of tables in #1523

[jangorecki]

@MichaelChirico
re: columns() - function to gather metadata about the columns, address of the column (so #641) is one of the metadata that can be collected. Bigger picture of columns can be found in master...jangorecki:information_schema#diff-f804b88946820485d304b5ab91817450R67
Currently tables() is focused on printing, so not really useful for processing metadata.

[MichaelChirico]

Got it. Looks cool.

[jangorecki]

Sorry, I simply didn't confirm it. I mean #718

[MichaelChirico]

@jangorecki could this be closed?