Specialized vcat and similar for AbstractWeights?

[oschulz]

When combining vectors/tables of samples, we currently lose weight-type information:

julia> vcat(FrequencyWeights(rand(3)), FrequencyWeights(rand(5)))
8-element Vector{Float64}:

julia> vcat(uweights(3), uweights(5))
10-element Vector{Int64}:

As combining/appending samples is a common operations, I think it would make sense to specialize things like vcat to preserve weight type.

[devmotion]

Seems reasonable to add definitions whenever it makes sense, and fall back to the generic definition otherwise. This came up also in https://github.com/TuringLang/MCMCChains.jl/pull/354.