rust: add find_bit_benchmark_rust module.

Microbenchmark protected by a config FIND_BIT_BENCHMARK_RUST,
following `find_bit_benchmark.c` but testing the Rust Bitmap API.

We add a fill_random() method protected by the config in order to
maintain the abstraction.

The sample output from the benchmark, both C and Rust version:

find_bit_benchmark.c output:
```
Start testing find_bit() with random-filled bitmap
[  438.101937] find_next_bit:                  860188 ns, 163419 iterations
[  438.109471] find_next_zero_bit:             912342 ns, 164262 iterations
[  438.116820] find_last_bit:                  726003 ns, 163419 iterations
[  438.130509] find_nth_bit:                  7056993 ns,  16269 iterations
[  438.139099] find_first_bit:                1963272 ns,  16270 iterations
[  438.173043] find_first_and_bit:           27314224 ns,  32654 iterations
[  438.180065] find_next_and_bit:              398752 ns,  73705 iterations
[  438.186689]
               Start testing find_bit() with sparse bitmap
[  438.193375] find_next_bit:                    9675 ns,    656 iterations
[  438.201765] find_next_zero_bit:            1766136 ns, 327025 iterations
[  438.208429] find_last_bit:                    9017 ns,    656 iterations
[  438.217816] find_nth_bit:                  2749742 ns,    655 iterations
[  438.225168] find_first_bit:                 721799 ns,    656 iterations
[  438.231797] find_first_and_bit:               2819 ns,      1 iterations
[  438.238441] find_next_and_bit:                3159 ns,      1 iterations
```

find_bit_benchmark_rust.rs output:
```
[  451.182459] find_bit_benchmark_rust:
[  451.186688] Start testing find_bit() Rust with random-filled bitmap
[  451.194450] next_bit:                       777950 ns, 163644 iterations
[  451.201997] next_zero_bit:                  918889 ns, 164036 iterations
[  451.208642] Start testing find_bit() Rust with sparse bitmap
[  451.214300] next_bit:                         9181 ns,    654 iterations
[  451.222806] next_zero_bit:                 1855504 ns, 327026 iterations
```

Here are the results from 32 samples, with 95% confidence interval.
The microbenchmark was built with RUST_BITMAP_HARDENED=n and run on a
machine that did not execute other processes.

Random-filled bitmap:
+-----------+-------+-----------+--------------+-----------+-----------+
| Benchmark | Lang  | Mean (ms) | Std Dev (ms) | 95% CI Lo | 95% CI Hi |
+-----------+-------+-----------+--------------+-----------+-----------+
| find_bit/ | C     |    825.07 |        53.89 |    806.40 |    843.74 |
| next_bit  | Rust  |    870.91 |        46.29 |    854.88 |    886.95 |
+-----------+-------+-----------+--------------+-----------+-----------+
| find_zero/| C     |    933.56 |        56.34 |    914.04 |    953.08 |
| next_zero | Rust  |    945.85 |        60.44 |    924.91 |    966.79 |
+-----------+-------+-----------+--------------+-----------+-----------+

Rust appears 5.5% slower for next_bit, 1.3% slower for next_zero.

Sparse bitmap:
+-----------+-------+-----------+--------------+-----------+-----------+
| Benchmark | Lang  | Mean (ms) | Std Dev (ms) | 95% CI Lo | 95% CI Hi |
+-----------+-------+-----------+--------------+-----------+-----------+
| find_bit/ | C     |     13.17 |         6.21 |     11.01 |     15.32 |
| next_bit  | Rust  |     14.30 |         8.27 |     11.43 |     17.17 |
+-----------+-------+-----------+--------------+-----------+-----------+
| find_zero/| C     |   1859.31 |        82.30 |   1830.80 |   1887.83 |
| next_zero | Rust  |   1908.09 |       139.82 |   1859.65 |   1956.54 |
+-----------+-------+-----------+--------------+-----------+-----------+

Rust appears 8.5% slower for next_bit, 2.6% slower for next_zero.

In summary, taking the arithmetic mean of all slow-downs, we can say
the Rust API has a 4.5% slowdown.

Suggested-by: Alice Ryhl <aliceryhl@google.com>
Suggested-by: Yury Norov (NVIDIA) <yury.norov@gmail.com>
Reviewed-by: Yury Norov (NVIDIA) <yury.norov@gmail.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Burak Emir <bqe@google.com>
Signed-off-by: Yury Norov (NVIDIA) <yury.norov@gmail.com>

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