MATLAB-like `unique` where you can recreate the original with indices

General Usage
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7

Yeah, here it’s a bit tricky because somehow the PooledArray constructor is type-unstable

julia> using PooledArrays, Test

julia> @inferred PooledArray(rand(1:3, 100))
ERROR: return type PooledVector{Int64, UInt32, Vector{UInt32}} does not match inferred return type PooledArray{Int64}

so you do need the find_ia function barrier (that had also tricked me initially into believing that find_ia was much more expensive).