rename alloc.col to set.alloc.col

[jangorecki]

for consistency

[MichaelChirico]

If we're going to change, I wouldn't use set.alloc.col which sounds odd.

set_allocated_columns or the like would be more expressive & help get at what it's doing

[shrektan]

A little off-topic, but I really wish one day there's a way to auto set alloc.col() to the data.table objects from RDS files...