lib/rand: Optimizations (#7964)

rand.secureSource.Uint64 no longer allocates. rand.String uses a
strings.Builder. Benchmark results on linux/amd64:

name            old time/op    new time/op    delta
SecureSource-8    69.1ns ± 3%    51.7ns ± 3%   -25.21%  (p=0.000 n=20+10)
String-8          2.66µs ± 2%    1.95µs ± 1%   -26.61%  (p=0.000 n=10+10)

name            old alloc/op   new alloc/op   delta
SecureSource-8     8.00B ± 0%     0.00B       -100.00%  (p=0.000 n=20+10)
String-8            288B ± 0%       32B ± 0%   -88.89%  (p=0.000 n=10+10)

name            old allocs/op  new allocs/op  delta
SecureSource-8      1.00 ± 0%      0.00       -100.00%  (p=0.000 n=20+10)
String-8            33.0 ± 0%       1.0 ± 0%   -96.97%  (p=0.000 n=10+10)

lib/rand/random_test.go

@@ -44,3 +44,10 @@ func TestRandomUint64(t *testing.T) {
 		}
 	}
 }
+
+func BenchmarkString(b *testing.B) {
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		String(32)
+	}
+}