Join can be used on all types

[elliotchance]

In a recent version, I introduced fmt.Stringer, see:

https://github.com/elliotchance/pie/blob/master/functions/float64s.go#L19-L20

mightBeString.String() will be replaced with a fmt.Sprintf("%v", mightBeString) for types that do not implement fmt.Stringer.

The functions that support this as tagged with an "S" in the Functions list. This means that Join could be used on all types and StringSliceType can be removed :)