return types of x^y

[StefanKarpinski]

As per this discussion, implement these return types for x^y:

Int.^Int :: Union(Int,Float)
Array{Int}.^Int :: Union(Array{Int},Array{Float})
Array{Int}.^Array{Int} :: Array{Float}
Int.^Array{Int} :: Array{Float}

[JeffBezanson]

In commit dda35c7.

[StefanKarpinski]

oops. yeah, just saw that.