[lazy] Skip over incompressible data

Every 256 bytes the lazy match finders process without finding a match,
they will increase their step size by 1. So for bytes [0, 256) they search
every position, for bytes [256, 512) they search every other position,
and so on. However, they currently still insert every position into
their hash tables. This is different from fast & dfast, which only
insert the positions they search.

This PR changes that, so now after we've searched 2KB without finding
any matches, at which point we'll only be searching one in 9 positions,
we'll stop inserting every position, and only insert the positions we
search. The exact cutoff of 2KB isn't terribly important, I've just
selected a cutoff that is reasonably large, to minimize the impact on
"normal" data.

This PR only adds skipping to greedy, lazy, and lazy2, but does not
touch btlazy2.

| Dataset | Level | Compiler     | CSize ∆ | Speed ∆ |
|---------|-------|--------------|---------|---------|
| Random  |     5 | clang-14.0.6 |    0.0% |   +704% |
| Random  |     5 | gcc-12.2.0   |    0.0% |   +670% |
| Random  |     7 | clang-14.0.6 |    0.0% |   +679% |
| Random  |     7 | gcc-12.2.0   |    0.0% |   +657% |
| Random  |    12 | clang-14.0.6 |    0.0% |  +1355% |
| Random  |    12 | gcc-12.2.0   |    0.0% |  +1331% |
| Silesia |     5 | clang-14.0.6 | +0.002% |  +0.35% |
| Silesia |     5 | gcc-12.2.0   | +0.002% |  +2.45% |
| Silesia |     7 | clang-14.0.6 | +0.001% |  -1.40% |
| Silesia |     7 | gcc-12.2.0   | +0.007% |  +0.13% |
| Silesia |    12 | clang-14.0.6 | +0.011% | +22.70% |
| Silesia |    12 | gcc-12.2.0   | +0.011% |  -6.68% |
| Enwik8  |     5 | clang-14.0.6 |    0.0% |  -1.02% |
| Enwik8  |     5 | gcc-12.2.0   |    0.0% |  +0.34% |
| Enwik8  |     7 | clang-14.0.6 |    0.0% |  -1.22% |
| Enwik8  |     7 | gcc-12.2.0   |    0.0% |  -0.72% |
| Enwik8  |    12 | clang-14.0.6 |    0.0% | +26.19% |
| Enwik8  |    12 | gcc-12.2.0   |    0.0% |  -5.70% |

The speed difference for clang at level 12 is real, but is probably
caused by some sort of alignment or codegen issues. clang is
significantly slower than gcc before this PR, but gets up to parity with
it.

I also measured the ratio difference for the HC match finder, and it
looks basically the same as the row-based match finder. The speedup on
random data looks similar. And performance is about neutral, without the
big difference at level 12 for either clang or gcc.

lib/compress/zstd_compress_internal.h

@@ -249,6 +249,13 @@ struct ZSTD_matchState_t {
      * This behavior is controlled from the cctx ms.
      * This parameter has no effect in the cdict ms. */
     int prefetchCDictTables;
+
+    /* When == 0, lazy match finders insert every position.
+     * When != 0, lazy match finders only insert positions they search.
+     * This allows them to skip much faster over incompressible data,
+     * at a small cost to compression ratio.
+     */
+    int lazySkipping;
 };
 
 typedef struct {