Split block compresser out of long range matcher

* `ZSTD_ldm_generateSequences()` generates the LDM sequences and
  stores them in a table. It should work with any chunk size, but
  is currently only called one block at a time.
* `ZSTD_ldm_blockCompress()` emits the pre-defined sequences, and
  instead of encoding the literals directly, it passes them to a
  secondary block compressor. The code to handle chunk sizes greater
  than the block size is currently commented out, since it is unused.
  The next PR will uncomment exercise this code.
* During optimal parsing, ensure LDM `minMatchLength` is at least
  `targetLength`. Also don't emit repcode matches in the LDM block
  compressor. Enabling the LDM with the optimal parser now actually improves
  the compression ratio.
* The compression ratio is very similar to before. It is very slightly
  different, because the repcode handling is slightly different. If I remove
  immediate repcode checking in both branches the compressed size is exactly
  the same.
* The speed looks to be the same or better than before.

Up Next (in a separate PR)
--------------------------

Allow sequence generation to happen prior to compression, and produce more
than a block worth of sequences. Expose some API for zstdmt to consume.
This will test out some currently untested code in
`ZSTD_ldm_blockCompress()`.

lib/compress/zstd_compress_internal.h

@@ -186,6 +186,7 @@ struct ZSTD_CCtx_s {
 
     seqStore_t seqStore;    /* sequences storage ptrs */
     ldmState_t ldmState;    /* long distance matching state */
+    rawSeq* ldmSequences;   /* Storage for the ldm output sequences */
     ZSTD_blockState_t blockState;
     U32* entropyWorkspace;  /* entropy workspace of HUF_WORKSPACE_SIZE bytes */