Add benchmarks comparing random_mod and random_bits

[dvdplm]

@tarcieri I noticed that generating random numbers using random_mod is slower than using random_bits repeatedly until the random number is below the given bound, like so:

            let bound = rng.next_u64();
            let bound_bits = bound.bits_vartime();

            let mut r = U1024::random_bits(&mut rng, bound_bits);
            while r >= bound {
                r = U1024::random_bits(&mut rng, bound_bits);
            }

The benchmarks show that random_mod is ~4x slower than random_bits.

It's not obvious to me why this is the case and perhaps it is to be expected. Is this something that is worth investigating further?

     Running benches/uint.rs (target/release/deps/uint-e0f5a60c64b6a49c)
bounded random/random_mod, U1024
                        time:   [533.35 ns 919.84 ns 1.4034 µs]
                        change: [-52.125% -13.878% +49.260%] (p = 0.64 > 0.05)
                        No change in performance detected.
Found 12 outliers among 100 measurements (12.00%)
  6 (6.00%) high mild
  6 (6.00%) high severe
bounded random/brute force, U1024
                        time:   [216.74 ns 229.78 ns 243.08 ns]
                        change: [-6.0962% -0.2957% +5.4555%] (p = 0.93 > 0.05)
                        No change in performance detected.
bounded random/brute force, U1024, small bound
                        time:   [28.588 ns 31.115 ns 33.674 ns]
                        change: [-8.3725% -0.7117% +8.0549%] (p = 0.87 > 0.05)
                        No change in performance detected.
bounded random/random_mod, U1024, small bound
                        time:   [63.862 ns 118.93 ns 204.96 ns]
Found 11 outliers among 100 measurements (11.00%)
  3 (3.00%) high mild
  8 (8.00%) high severe
bounded random/brute force, U1024, 512 bit bound low
                        time:   [102.46 ns 108.57 ns 114.95 ns]
                        change: [-6.2214% -0.2501% +5.6826%] (p = 0.93 > 0.05)
                        No change in performance detected.
bounded random/random_mod, U1024, 512 bit bound low
                        time:   [373.52 ns 825.42 ns 1.6169 µs]
Found 15 outliers among 100 measurements (15.00%)
  2 (2.00%) high mild
  13 (13.00%) high severe
bounded random/brute force, U1024, 512 bit bound hi
                        time:   [203.00 ns 216.22 ns 230.30 ns]
                        change: [-3.7858% +2.6058% +9.1138%] (p = 0.43 > 0.05)
                        No change in performance detected.
bounded random/random_mod, U1024, 512 bit bound hi
                        time:   [557.52 ns 729.76 ns 933.31 ns]
Found 8 outliers among 100 measurements (8.00%)
  1 (1.00%) high mild
  7 (7.00%) high severe