Deprecate vecbind, rbind, cbind

Also fixes typing issues in `vcat`/`rbind` due to missing columns being added as Float64s, and `rbind`/`vcat` now handle PooledData the same way `vcat` normally does (returning PooledDataVectors).

`vcat`/`rbind` no longer accept (unsplatted) Any arrays that happen to be filled with DataFrames.