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.go

@@ -0,0 +1,25 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package rands contains utility functions for randomness.
+package rands
+
+import (
+	crand "crypto/rand"
+	"encoding/hex"
+)
+
+// HexString returns a string of n cryptographically random lowercase
+// hex characters.
+//
+// That is, HexString(3) returns something like "0fc", containing 12
+// bits of randomness.
+func HexString(n int) string {
+	nb := n / 2
+	if n%2 == 1 {
+		nb++
+	}
+	b := make([]byte, nb)
+	crand.Read(b)
+	return hex.EncodeToString(b)[:n]
+}