Code replicated in compression and decompression moved to shared headers

`CHECK_F` macro moved to `error_private.h` (shared between `fse_compress.c` and `fse_decompress.c`). `ZSTD_limitCopy()` moved to `zstd_internal.h` (shared between `zstd_compress.c` and `zstd_decompress.c`). Erroneous build artefact `zstd.h` removed from repo.

lib/compress/zstd_compress.c

@@ -3749,17 +3749,6 @@ static size_t ZSTD_nextInputSizeHint(const ZSTD_CCtx* cctx)
     return hintInSize;
 }
 
-#ifndef ZSTD_DECOMPRESS_INTERNAL_H /* zstd_decompress.c has the same function */
-MEM_STATIC size_t ZSTD_limitCopy(void* dst, size_t dstCapacity, const void* src, size_t srcSize)
-{
-    size_t const length = MIN(dstCapacity, srcSize);
-    if (length > 0) {
-        memcpy(dst, src, length);
-    }
-    return length;
-}
-#endif
-
 /** ZSTD_compressStream_generic():
  *  internal function for all *compressStream*() variants
  *  non-static, because can be called from zstdmt_compress.c