tailcfg: optimize keyMarshalText

This function accounted for ~1% of all allocs by tailscaled.
It is trivial to improve, so may as well.

name              old time/op    new time/op    delta
KeyMarshalText-8     197ns ± 0%      47ns ± 0%  -76.12%  (p=0.016 n=4+5)

name              old alloc/op   new alloc/op   delta
KeyMarshalText-8      200B ± 0%       80B ± 0%  -60.00%  (p=0.008 n=5+5)

name              old allocs/op  new allocs/op  delta
KeyMarshalText-8      5.00 ± 0%      1.00 ± 0%  -80.00%  (p=0.008 n=5+5)

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

tailcfg/tailcfg_test.go

@@ -518,3 +518,13 @@ func TestEndpointTypeMarshal(t *testing.T) {
 		t.Errorf("got %s; want %s", got, want)
 	}
 }
+
+var sinkBytes []byte
+
+func BenchmarkKeyMarshalText(b *testing.B) {
+	b.ReportAllocs()
+	var k [32]byte
+	for i := 0; i < b.N; i++ {
+		sinkBytes = keyMarshalText("prefix", k)
+	}
+}