Progress deprecation of substitute() on LHS of :=

[MichaelChirico]

data.table/R/data.table.R

Lines 1133 to 1136 in 436bd6c

	#6033 revdep. Slowly deprecate in 1.17.0. Caller has given us `dt[, substitute(names(.SD))]` which means
	# jsub is actually substitute(names(.SD)) instead of just names(.SD)
	if (lhs %iscall% 'substitute')
	lhs = eval(lhs, parent.frame(), parent.frame())

Assigning to @ColeMiller1 who I think has the best context on what to do next here. Should we just add an appropriate warning in this branch?