This replaces NA with Nulls.null and NAtype with Nulls.Null.
The only breaking change is that null == null gives true, while NA == NA
gave NA (same for other comparison operators).

This has just been changed in Nulls.jl.

Now works with Nulls 0.1.0.

Rounding and transpose operations have been moved to Nulls,
functions from SpecialFunctions will have to be handled manually as we don't
want Nulls to depend on SpecialFunctions and keeping them in DataArrays
would be type piracy.

…hDropNull)

This allows collect(Nulls.skip(x)) to be equivalent to the old dropna(x)
for DataArray, but more generic. Also unexport the iterators, which are
an implementation detail.

Previously, convert() would create a DataArray{Union{T, Null}} containing
nulls which were not considered as missing, giving weird results in particular
with equality tests.

Constructing a DataArray{T} from an Array{Union{T, Null}} is tricky because
the field must be an Array{T}, yet conversion will fail in the presence of
nulls. Therefore, we need to allocate a copy and leave the null entries
uninitialized.

This prevents a conflict with CategoricalArrays.

…ay{>:Null})