Change the default type of ref

[bkamins]

Currently we have:

julia> x = PooledArray(zeros(Int, 10^6));
julia> copyto!(x, 1:10^6);
ERROR: You're using a PooledArray with ref type UInt8, which can only hold 255 values,
and you just tried to add the 256th reference.  Please change the ref type
to a larger int type, or use the default ref type (UInt32).

I think we should use UInt32 as the default as in practice limiting up to 255 levels by default is a small value. If someone wants such a limit to save memory I would have it as an opt-in rather than the default.

@quinnj - what does CSV.jl use as a default?

[quinnj]

CSV.jl uses UInt32; I agree we should just make UInt32 the default here