perf: New implemention for Diff()

[leilei3167]

Using set instead of nested for loops, the time complexity is reduced from O(n^2) to O(n)

[leilei3167]

@elliotchance please take a look

[leilei3167]

Results of the unit test:

$ go test -run ^TestDiff$ -v
=== RUN   TestDiff
=== RUN   TestDiff/BothEmpty
=== RUN   TestDiff/OnlyRemovedUnique
=== RUN   TestDiff/OnlyRemovedDuplicates
=== RUN   TestDiff/OnlyAddedUnique
=== RUN   TestDiff/OnlyAddedDuplicates
=== RUN   TestDiff/AddedAndRemovedUnique
=== RUN   TestDiff/AddedAndRemovedDuplicates
--- PASS: TestDiff (0.00s)
    --- PASS: TestDiff/BothEmpty (0.00s)
    --- PASS: TestDiff/OnlyRemovedUnique (0.00s)
    --- PASS: TestDiff/OnlyRemovedDuplicates (0.00s)
    --- PASS: TestDiff/OnlyAddedUnique (0.00s)
    --- PASS: TestDiff/OnlyAddedDuplicates (0.00s)
    --- PASS: TestDiff/AddedAndRemovedUnique (0.00s)
    --- PASS: TestDiff/AddedAndRemovedDuplicates (0.00s)
PASS
ok      github.com/elliotchance/pie/v2  0.196s