Code movements and next releases

[timholy]

Focusing just on the movement of code between repos (ignoring other bugfixes/features/doc improvements/etc), my thinking is:

• 0.7: moving code out of Images; use of OffsetArrays.centered here
• 0.8: move core functionality to JuliaArrays ArrayFiltering.jl (xref https://discourse.julialang.org/t/converting-between-axisarray-like-packages/52510/16?u=tim.holy)
• 0.8 or 0.9: benchmarking and LoopVectorization/KernelAbstractions/whatever