Error with ChainedVector with non-Vector storage

[mtsch]

Hi,
Since #106, ChainedVectors can't be indexed when their storage is any type other than Vector, as unsafe_copyto! is not defined on them (see MWE below). I ran into this when using Arrow.jl with chunked tables, which store the columns as ChainedVector{Arrow.Primitive}.

julia> using SentinelArrays
julia> x = ChainedVector([1:2, 3:4])
julia> x[1:2]
ERROR: MethodError: no method matching unsafe_copyto!(::Vector{Int64}, ::Int64, ::UnitRange{Int64}, ::Int64, ::Int64)
The function `unsafe_copyto!` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  unsafe_copyto!(::Array, ::Any, ::Array, ::Any, ::Any)
   @ Base array.jl:282
  unsafe_copyto!(::BitArray, ::Integer, ::Union{BitArray, Array}, ::Integer, ::Integer)
   @ Base bitarray.jl:456
  unsafe_copyto!(::Memory{T}, ::Any, ::Memory{T}, ::Any, ::Any) where T
   @ Base genericmemory.jl:128
  ...

Stacktrace:
 [1] getindex(x::ChainedVector{Int64, UnitRange{Int64}}, inds::UnitRange{Int64})
   @ SentinelArrays ~/.julia/packages/SentinelArrays/v0rcL/src/chainedvector.jl:202

[devmotion]

I opened #108.