Prysm: `IntersectionUint64`, `IntersectionInt64`, `IntersectionByteSlices` return union if more than 2 slices are provided

[terencechain]

🐞 Bug Report

Description

Pointed out by the Quantstamp team, the current function to return an intersection of input slices fails to return the intersection. Instead, it returns the union: https://play.golang.org/p/ewIKeH0sNoR

It works with two slices. But no more than that.

Implementation: https://github.com/prysmaticlabs/prysm/blob/master/shared/sliceutil/slice.go#L36

[kbak]

same bug occurs in IntersectionInt64()

[kbak]

also in IntersectionByteSlices(), see https://play.golang.org/p/rfqqq1FPA_N