Proposal: Add ElementsMatch, ElementsMatchBy

[senago]

I often need to get to know If two slices are comprised of the same set of elements.
There's a similar helper in testify package: ElementsMatch.

The proposal:

// ElementsMatch returns true if lists
// contain the same set of elements (including empty set).
// 
// If there are duplicate elements,
// the number of appearances of each of them in both lists should match.
func ElementsMatch[T comparable, Slice ~[]T](list1 Slice, list2 Slice) bool

// ElementsMatchBy returns true if lists
// contain the same set of elements' keys (including empty set).
// 
// If there are duplicate keys,
// the number of appearances of each of them in both lists should match.
func ElementsMatchBy[T any, K comparable](list1 []T, list2 []T, iteratee func(item T) K) bool

To implement ElementsMatch we could make use of Keyify.
To implement ElementsMatchBy some new code is needed.