ready Accumulator for v1.0

[oxinabox]

checks off accumulator.jl on https://github.com/JuliaCollections/DataStructures.jl/issues/479

One thing I am not sure about is the names nlargest and nsmallest those could be changed

[Krastanov]

In https://github.com/JuliaCollections/DataStructures.jl/issues/479#issuecomment-682449805 there is an admonition to not use push!(container, key, value), but here it says # inc! is preferred over push!, but we need to provide push! for the Bag interpreation. Just putting it out here in case this contradiction is important.