slices: update to current standard library version

Update x/exp/slices to the current standard library slices package,
while retaining the ability to use it with Go 1.18 through Go 1.20.

Note that this changes some of the sorting functions to use a
comparison function rather than a less function. We don't promise
backward compatibility in x/exp packages. Being compatible with the
Go 1.21 package seems more useful for people not yet using 1.21,
as it will make the transition to 1.21 easier.

The generated files were built using "go generate" with a GOROOT
that included CL 511660.

Fixes golang/go#61374

Change-Id: I4abfd9db92d553f554aec83d60f0c13fa56c1d8e
Reviewed-on: https://go-review.googlesource.com/c/exp/+/511895
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Eli Bendersky <eliben@google.com>

slices/cmp.go

@@ -0,0 +1,44 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package slices
+
+import "golang.org/x/exp/constraints"
+
+// min is a version of the predeclared function from the Go 1.21 release.
+func min[T constraints.Ordered](a, b T) T {
+	if a < b || isNaN(a) {
+		return a
+	}
+	return b
+}
+
+// max is a version of the predeclared function from the Go 1.21 release.
+func max[T constraints.Ordered](a, b T) T {
+	if a > b || isNaN(a) {
+		return a
+	}
+	return b
+}
+
+// cmpLess is a copy of cmp.Less from the Go 1.21 release.
+func cmpLess[T constraints.Ordered](x, y T) bool {
+	return (isNaN(x) && !isNaN(y)) || x < y
+}
+
+// cmpCompare is a copy of cmp.Compare from the Go 1.21 release.
+func cmpCompare[T constraints.Ordered](x, y T) int {
+	xNaN := isNaN(x)
+	yNaN := isNaN(y)
+	if xNaN && yNaN {
+		return 0
+	}
+	if xNaN || x < y {
+		return -1
+	}
+	if yNaN || x > y {
+		return +1
+	}
+	return 0
+}