.SD could be shallow copied when no 'by'

[arunsrinivasan]

Here's the relevant SO post.

When there's no by, using:

DT[, lapply(.SD, ...), .SDcols = ...]

need not deep copy .SD.

Note: It might be a bit tricky for cases like:

ans = DT[, .SD]
# or
ans = DT[, lapply(.SD, function(x) x)]

where if no copy is made, and a sub-assign by reference is done on ans, but still we can optimise cases where we're sure.

Or add the shallow copied attribute so that we could check it on sub-assign (as discussed already).

[arunsrinivasan]

More or less a duplicate of #617. When that's implemented, this'll be automatically taken care of.