Version attribute on service name

[roblabla]

We have version flags for cmds, but not for interfaces or service names. That'd be good to have.

interface meh is one, two, @version(2.0.0+) three {}

This looks kinda gross, but could work. Another possibility (although a fairly major departure):

@provides(one)
@provides(two)
@provides(three, 2.0.0+)
interface meh {}