Consider using binding sequence numbers rather than worlds for printing old objects

[Keno]

Right now we have:

julia> struct Foo
           x::Int
       end

julia> x = Foo(1)
Foo(1)

julia> Base.delete_binding(@__MODULE__, :Foo)

julia> struct Foo
           x::Int
       end

julia> x
@world(Foo, 36312:36315)(1)

In discussion with @JeffBezanson, we didn't like having the world age numbers exposed, because
we may want them to in theory be garbage collectable and compressible in the future. We discussed
that instead of world ages, we may want to use sequence numbers of the binding replace, e.g.
@partition(Foo, 1) or @partition(Foo, end-1).