Proposal for SharedFunctions.jl package for optional dependency management

[cstjean]

I like the idea, and this rule especially:

@quinnj

each generic function would have a sole “owner” package, in charge of providing documentation/maintenance, as well as being the sole package allowed to provide generic fallback definitions of the generic shared function (i.e. allowed piracy)

I do believe that this issue ought to be solved in base, though. There should be a way to define a LightGraphs.cartesian_product method without importing LightGraphs.