define emptymutable for all sets so that map, filter etc return right type

We should define Base.emptymutable for all out mutable AbstractSet subtypes.

Consider SortedSet

julia> filter(isodd, SortedSet((1,2,3,4)))
Set{Int64} with 2 elements:
  3
  1

But if we define:

Base.emptymutable(s::SortedSet{T}, ::Type{U}=T) where {T,U} = SortedSet{U}()

then we get

julia> filter(isodd, SortedSet((1,2,3,4)))
SortedSet{Int64,Base.Order.ForwardOrdering} with 2 elements:
  1
  3

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