External matchfinder API (#3333)

* First building commit with sample matchfinder

* Set up ZSTD_externalMatchCtx struct

* move seqBuffer to ZSTD_Sequence*

* support non-contiguous dictionary

* clean up parens

* add clearExternalMatchfinder, handle allocation errors

* Add useExternalMatchfinder cParam

* validate useExternalMatchfinder cParam

* Disable LDM + external matchfinder

* Check for static CCtx

* Validate mState and mStateDestructor

* Improve LDM check to cover both branches

* Error API with optional fallback

* handle RLE properly for external matchfinder

* nit

* Move to a CDict-like model for resource ownership

* Add hidden useExternalMatchfinder bool to CCtx_params_s

* Eliminate malloc, move to cwksp allocation

* Handle CCtx reset properly

* Ensure seqStore has enough space for external sequences

* fix capitalization

* Add DEBUGLOG statements

* Add compressionLevel param to matchfinder API

* fix c99 issues and add a param combination error code

* nits

* Test external matchfinder API

* C90 compat for simpleExternalMatchFinder

* Fix some @nocommits and an ASAN bug

* nit

* nit

* nits

* forward declare copySequencesToSeqStore functions in zstd_compress_internal.h

* nit

* nit

* nits

* Update copyright headers

* Fix CMake zstreamtest build

* Fix copyright headers (again)

* typo

* Add externalMatchfinder demo program to make contrib

* Reduce memory consumption for small blockSize

* ZSTD_postProcessExternalMatchFinderResult nits

* test sum(matchlen) + sum(litlen) == srcSize in debug builds

* refExternalMatchFinder -> registerExternalMatchFinder

* C90 nit

* zstreamtest nits

* contrib nits

* contrib nits

* allow block splitter + external matchfinder, refactor

* add windowSize param

* add contrib/externalMatchfinder/README.md

* docs

* go back to old RLE heuristic because of the first block issue

* fix initializer element is not a constant expression

* ref contrib from zstd.h

* extremely pedantic compiler warning fix, meson fix, typo fix

* Additional docs on API limitations

* minor nits

* Refactor maxNbSeq calculation into a helper function

* Fix copyright

lib/common/error_private.c

@@ -31,6 +31,7 @@ const char* ERR_getErrorString(ERR_enum code)
     case PREFIX(checksum_wrong): return "Restored data doesn't match checksum";
     case PREFIX(literals_headerWrong): return "Header of Literals' block doesn't respect format specification";
     case PREFIX(parameter_unsupported): return "Unsupported parameter";
+    case PREFIX(parameter_combination_unsupported): return "Unsupported combination of parameters";
     case PREFIX(parameter_outOfBound): return "Parameter is out of bound";
     case PREFIX(init_missing): return "Context should be init first";
     case PREFIX(memory_allocation): return "Allocation error : not enough memory";
@@ -51,6 +52,7 @@ const char* ERR_getErrorString(ERR_enum code)
     case PREFIX(seekableIO): return "An I/O error occurred when reading/seeking";
     case PREFIX(dstBuffer_wrong): return "Destination buffer is wrong";
     case PREFIX(srcBuffer_wrong): return "Source buffer is wrong";
+    case PREFIX(externalMatchFinder_failed): return "External matchfinder returned an error code";
     case PREFIX(maxCode):
     default: return notErrorCode;
     }