util/rands: add package with HexString func

We use it a number of places in different repos. Might as well make
one. Another use is coming.

Updates #cleanup

Change-Id: Ib7ce38de0db35af998171edee81ca875102349a4
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

util/rands/rands_test.go

@@ -0,0 +1,15 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package rands
+
+import "testing"
+
+func TestHexString(t *testing.T) {
+	for i := 0; i <= 8; i++ {
+		s := HexString(i)
+		if len(s) != i {
+			t.Errorf("HexString(%v) = %q; want len %v, not %v", i, s, i, len(s))
+		}
+	}
+}