factor::table: Optimise the size of the precomputed table

A too-large precomputed table actually slows down the program: we spend
time fetching it from disk and from memory (into the CPU's cache), and
larger prime factors are more unlikely to occur in random integers (prime p
occurs with probability ~1/p when sampling 64b numbers uniformly-at-random)

The new value was chosen after measuring the execution time (for all
integers between 2 and 10⁷) for a broad set of values:

| n    | time (s) |
|------|----------|
| 16   | 40.84    |
| 32   | 34.491   |
| 64   | 29.044   |
| 128  | 25.121   |
| 192  | 23.98    |
| 256  | 23.102   |
| 256  | 24.93    |
| 272  | 23.57    |
| 288  | 23.85    |
| 304  | 23.91    |
| 320  | 23.24    |
| 329  | 23.45    |
| 336  | 23.55    |
| 352  | 23.09    |
| 368  | 23.65    |
| 384  | 23.32    |
| 384  | 23.36    |
| 400  | 23.30    |
| 416  | 23.38    |
| 432  | 23.42    |
| 448  | 23.95    |
| 448  | 24.00    |
| 464  | 23.81    |
| 480  | 23.55    |
| 496  | 24.10    |
| 512  | 24.101   |
| 512  | 24.23    |
| 1027 | 29.864   |

src/uu/factor/build.rs

@@ -39,7 +39,7 @@ fn main() {
     let mut file = File::create(&Path::new(&out_dir).join("prime_table.rs")).unwrap();
 
     // By default, we print the multiplicative inverses mod 2^64 of the first 1k primes
-    const DEFAULT_SIZE: usize = 1027;
+    const DEFAULT_SIZE: usize = 320;
     let n = args()
         .nth(1)
         .and_then(|s| s.parse::<usize>().ok())