Implement empty / empty! as the truncate operation

[Nosferican]

Proposal to have these definitions implemented and added to the API

empty(df::AbstractDataFrame) = similar(df, 0)
empty!(df::AbstractDataFrame) = filter!(row -> false, df)

[bkamins]

I think the deprecation on empty! was long enough there that it is OK to add it back.
empty! will be implemented as (note that it is not for AbstractDataFrame as it may not be mutable)

empty!(df::DataFrame) = foreach(empty!, eachcol(df))

[bkamins]

see #2262