ambiguity in vcat

[bkamins]

We currently have the following design issue:

julia> vcat([1,2,3], PooledArray([1,2,3]))
ERROR: MethodError: vcat(::Vector{Int64}, ::PooledVector{Int64, UInt32, Vector{UInt32}}) is ambiguous.

julia> vcat(PooledArray([1,2,3]), [1,2,3])
ERROR: MethodError: vcat(::PooledVector{Int64, UInt32, Vector{UInt32}}, ::Vector{Int64}) is ambiguous.

Related to https://julialang.zulipchat.com/#narrow/stream/274208-helpdesk-.28published.29/topic/vcat.20dataframes.20with.20pooledarrays