Disallow mutable eltype in PooledArray

[bkamins]

I think we should disallow allowing mutable elements of PooledArray.

The reason of this proposal is the following problem:

julia> x = PooledArray([[1]])
1-element PooledArray{Array{Int64,1},UInt8,1,Array{UInt8,1}}:
 [1]

julia> push!(x[1], 2)
2-element Array{Int64,1}:
 1
 2

julia> x.invpool
Dict{Array{Int64,1},UInt8} with 1 entry:
  [1, 2] => 0x01

julia> x.invpool[[1,2]]
ERROR: KeyError: key [1, 2] not found

Alternatively we should change how invpool is handled if mutable objects are stored in PooledArray or explicitly warn that it is not allowed to mutate them.

[nalimilan]

It would indeed be safer. As an alternative maybe we could use an ObjectIdDict for mutable objects, but that probably more work than it's worth.

[bkamins]

Then, given we have tagged PooledArrays.jl as 1.0, maybe we just add a warning not to mutate the stored objects?

[quinnj]

Hmmm, in Arrow.jl, you're allowed to dict-encode anything you want, but then again, everything is assumed immutable, so this isn't really an issue. On the one hand, no one has ever brought this up, so I wonder if people tend to not do this anyway, or just don't run into it. A warning might be fine, just as a way to see if anybody might be doing some mutating unintentionally. But I think we should keep allowing mutable eltypes for now.