add SliceAsSetsable interface

[yflau]

I want to use the hashstructure to compute compatible hash of json schema, and first I use the yaml lib to unmarshal json schema because of its simplicity, the struct is something like this:

type Node struct {
Kind int
Content []*Nodes
}

The Kind == 2 stands for a Sequence and Kind == 4 stands for a Map, when compute hash, I want to set sliceAsSets to true for Map and false to Sequence, but tag solution can not do this, so I want to add a interface to solve it.