copyto! definition mismatch with Base

[tomaklutfu]

In these functions, https://github.com/JuliaDiffEq/RecursiveArrayTools.jl/blob/38e37ddf1c6de419e18ed3d89b1717670d2138fb/src/array_partition.jl#L106-L123
destination is not returned. This breaks Base's assumption in collect and ABI of copyto!.
https://github.com/JuliaLang/julia/blob/67cdc550a0c5d615c4071b7fae12caaf85d01e30/base/array.jl#L560
https://github.com/JuliaLang/julia/blob/67cdc550a0c5d615c4071b7fae12caaf85d01e30/base/array.jl#L576-L577

[tomaklutfu]

Thanks for quick fix.