Having a `slices.Contains` check within a `for` loop has a complexity of
`O(n*m)`, with a worst case scenario of `O(n^2)` for when both
collections have the same size.
This is because `slices.Contains` internally runs just a regular loop,
checking for equality element by element.
Instead, this diff changes every occurence of this pattern to converting
the collection we compare against to a set, and then running a faster
lookup against it.
Michael Manganiello
80d7cf0b65