Rename "External Matchfinder" to "Block-Level Sequence Producer" (#3484)

* change "external matchfinder" to "external sequence producer"

* migrate contrib/ to new naming convention

* fix contrib build

* fix error message

* update debug strings

* fix def of invalid sequences in zstd.h

* nit

* update CHANGELOG

* fix .gitignore

tests/external_matchfinder.h

@@ -27,8 +27,8 @@ typedef enum {
     EMF_INVALID_LAST_LITS = 8
 } EMF_testCase;
 
-size_t zstreamExternalMatchFinder(
-  void* externalMatchState,
+size_t zstreamSequenceProducer(
+  void* sequenceProducerState,
   ZSTD_Sequence* outSeqs, size_t outSeqsCapacity,
   const void* src, size_t srcSize,
   const void* dict, size_t dictSize,