Symbol hierarchy and reorganization

[johnnychen94]

Currently, a lot of symbols are directly reexported by its sub-packages without much organization, and that contributes to the long list of exported symbols:

julia> names(Images)
657-element Array{Symbol,1}:

Organizing these in a hierarchical and meaningful way is definitely a thing that we need to do for the 1.0 release.

I don't have a detailed proposal right now, but the basic idea is to introduce several submodules and organize them in a way like how scikit-image does:

Images.Core
Images.Utils
Images.IO
Images.Draw
Images.Measure # ImageQualityIndexes, ImageDistances
Images.Morphology
Images.Restoration # denoise, deblur, etc
Images.Transformation
# etc

[timholy]

Worth thinking about. I do worry about difficult-to-categorize functions, though. In the modern world I've moved to "flatter" organizations but that usually is because of a search ability. I wonder if we need to improve discoverability of methods by other mechanisms?