Defines ad-hoc equality operators for arbitrary SequenceTypes of Equatable elements of the same type. Seqeunces are considered equivalent iff they contain the same number of elements in the same order.
In practice this is most useful when testing custom collections against some reference collection (e.g. an array).
import Equality
let seq = CustomSequence(1, 2, 3)
assert(seq == [1, 2, 3])